This C program checks an input number and replies with the result after checking if the number is prime.

prime_or_not.c

#include <stdio.h>

int main() {
  int number;
  printf("Enter your number:\n");
  scanf("%d", &number);
  if (number > 3){ // 1,2,3 are prime for sure; verification starts for bigger numbers;
    for (int i = 2; i<number; i++){
      if (number % i == 0){
        printf("The number is not a prime number.\n");
        break;
      }
      else {
        printf("The number is prime.\n");
        break;
      }
    }
  } else {
    printf("The number is prime.\n");
  }
  return 0;
}