I think Answer is (B)
Since c is passed by value and x is passed by reference, all functions will have same copy of x, but different copies of c.
f(5, 5) = f(x, 4)*x = f(x, 3)*x*x = f(x, 2)*x*x*x = f(x, 1)*x*x*x*x = 1*x*x*x*x = x^4
Since x is incremented in every function call, it becomes9 after f(x, 2) call. So the value of expression x^4 becomes 9^4 which is 6561