#include <stdio.h>

int math(int a, int b, char selection) {

    int result;

    switch (selection) {
        case 'a':
        result = a + b;
        break;
        case 's':
        result = a - b;
        break;
        case 'm':
        result = a * b;
        break;
        case 'd':
        result = a / b;
        break;
    }

    return result;
}

int main() {
    int num1 = 0;
    int num2 = 0;
    int result = 0;
    char selection;

    printf("Enter first number: ");
    scanf("%i", &num1);

    printf("Enter second number: ");
    scanf("%i", &num2);

    printf("n[a] Addn[s] Subtractn[m] Multiplyn[d] DividennWhich one: ");
    scanf("%c", &selection);

    result = math(num1, num2, selection);
    printf("%i", result);

    return 0;
}

printf("Enter second number: ");
scanf("%i", &num2);

you can't see that, but it's there
                         V
> Enter second number: 3n

scanf("%c", &selection);

scanf(" %c", &selection);

printf("n[a] Addn[s] Subtractn[m] Multiplyn[d] DividennWhich one: ");
int ret = scanf("%c", &selection);

printf("Scanf I think failed. Return value is: %d. Selection is: %c (%d)n",
        ret, selection, selection);

Scanf I think failed. Return value is: 1. Selection is:
(10)