Format specifier %s in not showing result

I am learning the C programming language. I am doing homework with C programming language. But I am getting a problem. The result is not printing. I am using format specifier %s. Here is my code below. Please have a look.

#include <stdio.h>

int gradeCalculation(int marks) {
  switch (marks) {
    case 0 ... 49:
      return 'F';
      break;
    
    case 50 ... 59:
      return 'D';
      break;

    case 60 ... 69:
      return 'C';
      break;

    case 70 ... 79:
      return 'B';
      break;

    case 80 ... 89:
      return 'A';
      break;

    case 90 ... 100:
      return 'A+';
      break;

    default:
      return 'Marks is not valid!';
  }
}

int main() {
  int marks;

  printf("Marks: ");
  scanf("%d", &marks);

  printf("Grade: %s\n", gradeCalculation(marks));

  return 0;
}

How to solve my problem? What specifier should I use to get the expected result?