My answer would probably be D because the ledge by A looks a little to small.
If this question is from achiever i dont give a crap. Achievers PAT is damn awful and there is often more than one answer, even after using their explanation.
The answer is A. The little ledge that sticks out is not centered at all. It is the same dimensions as the cut out box above it. In the answer D, they have the ledge centered.
Edit : But you are right it does look to short.... stupid question.