what is the complexity of the following c Function ?

double foo (int n) {
    int i;
    double sum;
    if (n==0) return 1.0;
    else {
        sum = 0.0;
        for (i =0; i<n; i++)
        sum +=foo(i);
        return sum;
    }
}

Please don't just post the complexity can you help me in understanding how to go about it .

EDIT: It was an objective question asked in an exam and the Options provided were 1.O(1) 2.O(n) 3.O(n!) 4.O(n^n)