Reverse array content using recursion (C Language)

This:

char str;

gets(str);

is undefined behavior, and should give compiler warnings (or even errors) that you are ignoring.

gets() requires a pointer to an array of characters, you're giving it the value of a single character, which is not the same thing at all. It will interpret the undefined value of that character as an address, and store the input text there.

It should be:

int main()
{
  char line[128];

  printf("\nEnter an array of characters: ");
  if(fgets(line, sizeof line, stdin) != NULL)
  {
    const size_t len = strlen(line);
    printf("The reversed array of characters is = %s\n", reverseAr(line, len));
  }
  return 0;    
}

the return type of strlen() is not int, it's size_t so you should use that, and change reverseAr() accordingly.

This is the working code for reversing string

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h> 
#include <string.h> 
#include <math.h> 

int reverseAr(char ar[], int , int);

int main()
{
    char str[128];
    int len;

    printf("\nEnter an array of characters: ");

    if(fgets(str, sizeof(str), stdin) != NULL)
    {
        len = strlen(str);
        reverseAr(str, 0, len-1);
    }
    printf("The reversed array of characters is = %s\n", str );
    system("pause");
    return 0;
}

int reverseAr(char ar[], int first, int last)
{
    char temp = ar[first];
    ar[first] = ar[last];
    ar[last] = temp;

    if (first < last) 
    {
        reverseAr(ar, ++first, --last);
    }
    else
    {
        return 0;
    }
}

I think the problem is how you print the result:

void reverseAr(char ar[], int n);
printf("The reversed array of characters is = %s\n", reverseAr(str, len));

printf expects a string and you are passing it void...

You should replace that with:

reverseAr(str, len-1);
printf("The reversed array of characters is = %s\n", str);

You will notice I am passing len-1 insted of len to reverseAr that is because you want to leave the last character in the string ('\0') alone, and only switch the rest of the characters. Ultimately you want to fix the reverseAr function as well:

void reverseAr(char ar[], int i, int n)
{
    char temp = ar[i];
    ar[i] = ar[n];
    ar[n] = t;

    if (i < n) 
    {
        reverseAr(ar, i++, n - 1);
    }
}

And call it a differently: reverseAr(str,0,len-1);

You can use a pointer instead of using the array name to increment (as ++ar is not valid) in the reverseAr function.

The following code should work:

/* In Main */

int main()
{
        char str[128];
        int len;

        printf("\nEnter an array of characters: ");
        gets(str);    /* use fgets instead */

        len = strlen(str) - 1;  /* length is one less than the strlen */

        printf("The reversed array of characters is = %s\n", reverseAr(str, len));

        return 0;
}

void reverseAr(char *ar, int n)
{
        /* Base case */
        if (ar > (ar + n))
        {
            return;
        }
        char temp = *ar;
        *ar = *(ar + n);
        *(ar + n) = temp;
        reverseAr(ar + 1, n - 1);
    
}