This question is asking about the constructor for the Vector class. Easiest thing to do then is look it up in the API.
The constructor says:
Vector (int initialCapacity, int capacityIncrement)
So you know it starts with 5 and will increase by 10. Now the next question is when does it increase to 10, when you add the 5th element or the 6th element. Again look at the API:
capacityIncrement - the amount by which the capacity is increased when the vector overflows.
So it will increase when it overflows, which makes sense from a efficiency standpoint. That means when the 6th element is added, it will increase 10 more elements making C the correct answer as it will now have 15 elements.
Bill