More Simple

#include <stdio.h>
int main()
{
  int n, d, larger, smaller, diff;
  
  printf("Enter a fraction (nn/nn): ");
  scanf("%d/%d", &n, &d);
  
  if (d == 0)
  {
    printf("Error: Denominator cannot be zero.\n");
    return 1;
  }
  printf("%d/%d = ", n, d);
  larger = n > d ? n : d;
  smaller = n < d ? n : d;
  diff = larger - smaller;
  while (diff != larger)
  {
    larger = smaller > diff ? smaller : diff;
    smaller = smaller == larger ? diff : smaller;
    diff = larger - smaller;
  }
  if (diff > 1)
    printf("%d/%d\n", n / diff, d / diff);
  else
    printf("%d/%d\n", n, d);
  return 0;
}