What is the space complexity of the recursive implementation used to convert a decimal number to its binary equivalent?

#include<stdio.h>
int arr[31], len = 0;
void recursive_dec_to_bin(int n)
{
      if(n == 0 && len == 0)
      {
          arr[len++] = 0;
          return;
      }
      if(n == 0)
         return;
      arr[len++] = n % 2;
      recursive_dec_to_bin(n/2);
}

A. O(1)

B. O(n)

C. O(n²)

D. O(logn)

Answer: Option D