4 of 4 added 10 characters in body

C

Simply by using recursion instead of loops. This particular version only works on characters between '0' and 'z', but that's just to keep the stack from overflowing too often. Supporting all of ASCII.

#include <stdio.h>

void walk(char *s, char m){
    if (*s == '\0') return;
    if (*s == m) putchar(m);
    walk(s+1,m);
}

void sort(char *s, char m){
  walk(s,m);
  if (m<127) sort(s,m+1);
}

int main(int argc, char**argv){
  if (argc > 1) {
    printf("%s: ",argv[1]);
    sort(argv[1],1);
    printf("\n");
  }
}

Sample run:

$ ./a.out  thingamabob
thingamabob: aabbghimnot
$ ./a.out  supercalifragilisticexpalodoucious
supercalifragilisticexpalodoucious: aaacccdeefgiiiiilllooopprrssstuuux
$ ./a.out  "The quick red fox jumped over the lazy brown dog"
The quick red fox jumped over the lazy brown dog:          Tabcdddeeeeefghhijklmnoooopqrrrtuuvwxyz

Too easy, really.