string reverse using recursion

If you had tried to use a debugger, you would see that t is going mad on second iteration. This is because after you have copied string into b you forgot to insert \0 symbol at the end (position with index t-1). This causes t become literally anything on the next iteration because of strlen() needs a null-terminating string and it results in an undefined behaviour as mentioned in docs:

The behavior is undefined if str is not a pointer to a null-terminated byte string

So a quick fix is as folows:

...
for (int i=0;i<t-1;i++)
{
    b[i]=string[i];
}
b[t-1] = '\0';
reverse(b);
...

And as already mentioned in comments by @LPs : change scanf("%s",&a); to scanf("%199s",a); (199 because we need to leave a space for '\0' at the end, thanks to @RoadRunner for noticing that)

Note: take a look at strncpy_s (if you use C11) and use it instead of that for loop:

printf("%c",string[t-1]);
strncpy_s(b, 200, string, t-1);       // 200 because char b[200]
reverse(b);

or strncpy:

printf("%c",string[t-1]);
strncpy(b, string, t-1); 
b[t-1] = '\0';
reverse(b);

Still another approach is not to copy:

else
{
    string[t-1] = '\0';    // you don't need array 'b' at all
    reverse(string);
}

And the simpliest way is just to use a loop:

for (int i = strlen(string) - 1; i >= 0; --i)
{
    printf("%c", string[i]);
}

You mix up two approaches, i.e. recursive and loop, in one function. Further, for me it remains unclear whether you want to change the input string to a reverse order, or if you just want to print it out in reverse order. Anyway, see a solution that provides loop bases reverse printing and recursion based reverse printing; hope it helps in understanding the steps the code such goes through:

#include <stdio.h>
#include <string.h>

void reverse_loop(char string[]) {
    long i = strlen(string) - 1;
    while (i >= 0)
        printf("%c",string[i--]);
}

void reverse_recursive (char string[]) {
    if (string[0] != '\0') {
        reverse_recursive(&string[1]);
        printf("%c", string[0]);
    }
}

int main()
{
    char a[200] = { "some test string" };
    reverse_recursive(a);
    return 0;
}

The easiest way to reverse a string using recursion is like this:

    // C++ program to reverse a string using recursion
# include <stdio.h>
# include<string>
using namespace std;
 
string in="";
/* Function to print reverse of the passed string */
void reverse(char *str)
{
   if (*str)
   {
       reverse(str+1);
       in=in+*str;   /* If you want to save the reversed string */
       printf("%c", *str);  /*If you just want to print the reversed string */
   }
}
 
/* Driver program to test above function */
int main()
{
   char a[] = "Reversing a string using recursion";
   reverse(a);
   printf("\n%s\n",in.c_str());
   return 0;
}
    /* output : noisrucer gnisu gnirts a gnisreveR */

Explanation:

Recursive function (reverse) takes string pointer (str) as input and calls itself with next location to passed pointer (str+1). Recursion continues this way, when pointer reaches ‘\0’, all functions accumulated in stack print char at passed location (str) and return one by one.

Time Complexity: O(n)