(C) How to make a function return a string from a loop?

Question

C++ / C#

I recently started learning C (I work in VisualStudio 13)
How can I fix the loop so that it writes intermediate values ​​to a string and the function returns the finished string (or ...?). Perhaps there is a better solution to the problem.
I type 841 into the console, it returns:
>Sum = 13 // OK
I want it to return:
>Sum = 8 + 4 + 1 = 13

#include <stdio.h>
#include <stdlib.h>

unsigned Sum(unsigned n); // Functions' prototype
void main() {
  unsigned n;
  printf_s("Please, print --> ");
  scanf_s("%u", &n);
  printf_s("Sum=%u\n", Sum(n));
  system("pause");
}

unsigned Sum(unsigned n) {
  unsigned s = 0; // iteration(s)
  while (n > 0) {
    s+= n % 10;
    n /= 10;
  }
  return s; 
}

Answer 1

#include <stdio.h>
#include <stdlib.h>

void Sum(unsigned n); // Functions' prototype
void main() {
    unsigned n;
    printf_s("Please, print --> ");
    scanf_s("%u", &n);
    Sum(n);
    system("pause");
}

void Sum(unsigned n) {
    unsigned s = 0; // iteration(s)
    printf_s("Sum = ");
    while (n > 0) {
        printf("%d + ",n % 10);
        s+= n % 10;
        n /= 10;
    }
    printf_s("\b\b= %d\n",s);
}

Answer 2

Options
1) Pass a pointer to an array or a string to the function, and write intermediate values ​​to this array at each iteration of the loop
2) Create a global variable, and the function writes to it - not kosher
3) Create a structure that will contain the result and intermediate values ​​- fill it in and return it (or do it by pointer/reference)
4) Variations with an array, but the last number is the result.
Choose.