c program closed

Question

Explain how to evaluate it

how to evaluate recursion within loop

Consider the following C function:
int fun (int n)
{
int x=1, k;
if (n= =1) return x;
for (k=1; k < n; ++k)
x = x + fun(k) * fun(n – k);
}
The return value of fun(5) is __________.
(a) 0 (b) 26
(c) 51 (d) 71