An inline function is expanded duringa.run-timeb.end of the programc.never expandedd.compile-time

An inline function is expanded during compile-time. So, the correct answer is d. compile-time.

Here's a step-by-step explanation:

1. An inline function is a function that is expanded in line when it is called. When the inline function is called whole code of the inline function gets inserted or substituted at the point of inline function call.

2. This substitution is performed by the compiler at compile time.

3. The main advantage of this is that it reduces the function call overhead.

4. So, the inline function is expanded during compile-time, not at run-time or the end of the program. It's also not correct to say that it's never expanded.