8088 assembly, IBM PC DOS, 35 34 bytes

Machine code:

be81 00ad 80ec 308a f433 db8a c8d0 e973 0143 75f9 3af3 7504 b40e cd10 403c 7e7e e8c3

Listing:

BE 0081     MOV  SI, 081H   ; address of DOS PSP command line

AD          LODSW           ; load space (20H) and value from command line 

80 EC 30    SUB  AH, '0'    ; convert to numeric value 

8A F4       MOV  DH, AH     ; AL = counter, DH = target value

        CHR_LOOP: 

33 DB       XOR  BX, BX     ; clear bit counter 

8A C8       MOV  CL, AL     ; current char to CL for test 

        BIT_LOOP: 

D0 E9       SHR  CL, 1      ; shift LSB to CF 

73 01       JNC  EBIT_LOOP  ; if LSB = 0, don't increment 

43          INC  BX         ; increment '1' bit counter 

        EBIT_LOOP: 

75 F9       JNZ  BIT_LOOP   ; loop until CL = 0 

3A F3       CMP  DH, BL     ; is current char the target value? 

75 04       JNE  ECHR_LOOP  ; if not, do not display 

B4 0E       MOV  AH, 0EH    ; BIOS write char function 

CD 10       INT  10H        ; write AL to screen 

        ECHR_LOOP: 

40          INC  AX         ; increment char loop 

3C 7E       CMP  AL, 126    ; is char <= 126? 

7E E8       JLE  CHR_LOOP   ; if so, keep looping 

C3          RET             ; return to DOS

Standalone PC DOS executable program, input number from command line. Output is displayed to console window.

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Jelly, 8 bytes

ØṖOB§=ʋƇ

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ØṖ       printable ascii character list

  OB     to binary

    §    popcount

     =   equal to input?

      ʋƇ filter (implicitly output)

Japt, 9 bytes

;EƶXc¤è1

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;EƶXc¤è1     :Implicit input of integer U

;E            :Printable ASCII

  Æ           :Filter each X

   ¶          :Test U for equality with

    Xc        :  Character code of X

      ¤       :  To binary string

       è1     :  Count the 1s

05AB1E, 8 bytes

žQʒÇbSOQ

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Explanation

žQ        # push the printable ascii characters

  ʒ       # filter, keep elements whose

   Ç      # character code

    b     # converted to binary

     SO   # has a digit sum

       Q  # equal to the input

Python 2, 62 bytes

lambda n:[chr(i)for i in range(32,127)if bin(i).count('1')==n]

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JavaScript (Node.js),  70  69 bytes

n=>(g=x=>x>>7?'':Buffer((h=x=>x&&x%2+h(x>>1))(x)-n?0:[x])+g(x+1))(32)

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Wolfram Language (Mathematica), 70 bytes

FromCharacterCode/@Select[32~Range~126,s=#;Tr@IntegerDigits[#,2]==s&]&

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Python 2, 69 bytes

lambda n:[chr(i)for i in range(32,127)if sum(map(int,bin(i)[2:]))==n]

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Gaia, 10 bytes

₵R⟪¤cbΣ=⟫⁇

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		| implicit input, n

₵R		| push printable ascii

  ⟪	⟫⁇	| filter the list where:

   ¤cbΣ		| the sum of the code point in binary

       =	| is equal to n

Java 8, 131 bytes

Returns a java.util.stream.Stream<String>

n->java.util.stream.IntStream.range(32,127).filter(i->Long.toBinaryString(i).chars().map(c->c-48).sum()==n).mapToObj(c->(char)c+"")

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Using HashSet, 135 bytes. Returns a Set<Object>:

n->new java.util.HashSet(){{for(int i=31;i++<126;add(Long.toBinaryString(i).chars().map(c->c-48).sum()==n?(char)i+"":""),remove(""));}}

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Dyalog APL Extended, 24 bytes

⎕ucs a⌿⍨(+⌿2⊤a←32…126)∘=

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25 bytes in dzaima/APL:

⎕ucs a⌿⍨(+/¨2⊤¨a←32…126)=

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PowerShell, 83 bytes

param($n)[char[]](32..126|?{([convert]::ToString($_,2)|% t*y|group)[1].count-eq$n})

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Takes input $n, constructs a range from 32 to 126 and pulls out those numbers where |?{}: the number, converted ToString in base 2; converted toCharArray; grouped into 0s and 1s; taking the [1] index of that grouping; taking the .count thereof, and checking that it's -equal to our input $number. Those numbers are then cast as a char-array and left on the pipeline. Output is implicit, with newlines between elements.

Perl 5 -a, 50 bytes

map{(sprintf'%b',$_)=~y/1//-$F[0]||say chr}32..126

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Charcoal, 10 bytes

Φγ⁼Σ↨℅ι²Ｉθ

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 γ          Predefined ASCII characters

Φ           Filtered by

      ι     Current character's

     ℅      ASCII code

    ↨       Converted to base

       ²    Literal 2

   Σ        Summed

  ⁼         Equals

         θ  First input

        Ｉ   Cast to integer

            Implicitly printed

PHP, 72 bytes

for($x=31;$x++<126;)echo$argn==count_chars(decbin($x),1)[49]?chr($x):'';

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Ruby, 48 bytes

->n{(' '..?~).select{|x|x.ord.digits(2).sum==n}}

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Excel (2016 or later), 76 bytes

=CONCAT(IF(LEN(SUBSTITUTE(DEC2BIN(ROW(32:126)),0,))=A1,CHAR(ROW(32:126)),""))

Takes input from A1, outputs in whatever cell you put this formula. This is an array formula, so you need to press Ctrl-Shift-Enter to input it. The "2016 or later" is because it needs the CONCAT function (the deprecated CONCATENATE won't take an array as argument).

C# (Visual C# Interactive Compiler), 96 bytes

n=>Enumerable.Range(32,95).Where(x=>Convert.ToString(x,2).Count(c=>c==49)==n).Select(x=>(char)x)

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Brachylog, 7 bytes

∈Ṭ&ạhḃ+

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A predicate which functions as a generator, takes input through its output variable, and produces each character through its input variable. Because Brachylog.

           The input variable (which is an element of the output)

∈          is an element of

 Ṭ         the string containing every printable ASCII character

  &        and the input

   ạh      converted to a codepoint

     ḃ     converted to a list of binary digits

      +    sums to

           the output variable (which is the input).

J, 31 bytes

a.#~&(95{.32}.])]=1#.2#:@i.@^8:

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• 
  2#:@i.@^8: produces the binary numbers 0 through 255 (2 ^ 8 is 256)


• 
  1#. sums each one


• 
  ]= produces a binary mask showing where the sum equals the original input


• 
  a.#~ mask uses that binary mask to filter J's full ascii alphabet a.
  


• 
  &(95{.32}.]) but before doing so take only elements 32...126 from both the alphabet and the mask

C++ (gcc), 124 bytes

#import<string>

std::string a(int n){std::string f;for(int i=30,j,k;++i<127;j=n){k=i;while(k)j-=k&1,k/=2;f+=i*!j;}return f;}

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Iterates through the printable character codes, finds those that match the number of 1 bits desired, returns the string containing those characters concatenated.

Perl 6, 41 bytes

{chrs grep *.base(2).comb.sum==$_,^95+32}

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Anonymous code block that takes a number and returns a string of valid characters.