Regarding number 2 (again):

  unsigned sum(const unsigned char *s) {
    unsigned result = 0;
    size_t length = strlen(s);
    for (size_t i=0; i < length; i++) {
      result += s[i];
    }
    return result;
  }

But it could be written as:

  unsigned sum(const unsigned char *s) {
    unsigned result = 0;
    size_t length = strlen(s);
    for (size_t i=0; i < length; ++i) // prefix increment should be quicker.
      result += s[i];
    return result;
  }

Regarding number 2: Loop-invariant strlen():

I would write the loop as the optimised version first hand, especially if there were nested loops.

No. 5 caught me out, but I'm not surprised.

I got 0/6; glad I read this.