Program:

#include <stdio.h>

int main()
{
    int x;

    printf("Enter an integer number: ");
    scanf("%d", &x);

    if (x == 0)
        printf("%d", 0);
    else if (x % 9 == 0)
        printf("%d", 9);
    else
        printf("%d", x % 9);

    printf("\n");
 
   return 0;
}

Output:

RUN 1:
Enter an integer number: 123
6

RUN 2:
Enter an integer number: 98751
3

RUN 3:
Enter an integer number: 13
4

RUN 4:
Enter an integer number: 19082
2