What is the time complexity of the following recursive implementation used to find the length of the string?

#include<stdio.h>
int recursive_get_len(char *s, int len)
{
      if(s[len] == 0)
        return 0;
      return 1 + recursive_get_len(s, len+1);
}
int main()
{
      char *s = "abcdef";
      int len = recursive_get_len(s,0);
      printf("%d",len);
      return 0;
}

A. O(1)

B. O(n)

C. O(n²)

D. O(n³)

Answer: Option B