What is the time complexity of the following recursive implementation used to find the sum of the first n natural number

[answerhappygod]

#include<stdio.h>
int recursive_sum(int n)
{
if(n == 0)
return 0;
return n + recursive_sum(n - 1);
}
int main()
{
int n = 5;
int ans = recursive_sum(n);
printf("%d",ans);
return 0;
}
a) O(1)
b) O(n)
c) O(n2)
d) O(n3)