Тут всё Ясно

#include <stdio.h>

int largest_prime_divisor(int a) {
    if (a <= 1) {
        return -1;
    }

    int divisor = 2;
    while (divisor * divisor <= a) {
        if (a % divisor == 0) {
            a /= divisor;
        } else {
            divisor++;
        }
    }

    return a;
}

int main() {
    int a;
    printf("Enter a number: ");
    scanf("%d", &a);

    int result = largest_prime_divisor(a);

    if (result == -1) {
        printf("n/a\n");
    } else {
        printf("Largest prime divisor: %d\n", result);
    }

    return 0;
}

Пример работы программы:

Enter a number: 48
Largest prime divisor: 3

Enter a number: 17
Largest prime divisor: 17

Enter a number: 100
Largest prime divisor: 5