Explanation:
The given function is a recursive function that calculates the product of two numbers, x and y. Let's break down the code and understand how it works.

1. Base Case:
The function starts with a base case that checks if the value of y is equal to 0. If it is, the function returns 0. This is the stopping condition for the recursion.

2. Recursive Case:
If the base case is not satisfied, the function calls itself recursively with the parameters x and y-1. In each recursive call, the value of y is reduced by 1 until it reaches 0.

3. Return Value:
In each recursive call, the function multiplies the value of x with the result of the recursive call (i.e., fun(x, y-1)). This is done recursively until the base case is reached.

4. Example:
Let's take an example to understand how the function works. Suppose we have x = 3 and y = 4.

- The first call to the function is fun(3, 4).
- Since y is not 0, the function goes to the recursive case and calls fun(3, 3).
- Again, y is not 0, so the function calls fun(3, 2).
- This process continues until y becomes 0. At that point, the base case is satisfied, and the function returns 0.
- Now, in the previous recursive call, the value of y is 1. So, the function returns x * fun(x, y-1), i.e., 3 * 0 = 0.
- Similarly, in the initial call, the function returns x * fun(x, y-1), i.e., 3 * 0 = 0.

5. Final Result:
In the end, the function returns the product of x and y, which is 3 * 4 = 12.

Therefore, the correct answer is option 'C' (x * y), as the function calculates the product of the two input numbers using recursion.