No matter what, "Couldn't we leave before noon?" simply means the speaker is asking to leave before the noon i.e. 12 o'clock.
Adding clarity to the context, the former sentence talks about leaving at 1400 hr. what the speaker thinks is quite late for some reason.
If your teacher says that the answer is 'D' i.e. if they could leave the room, the only possibility I see is the time that sentence is spoken is the moment to leave for the trip which must be before the noon (what the speaker is concerned about).
However, to me, option D is an odd thing out! :) That's the only option that does not speak about the time and that's what the question is all about!
Your teacher is the person to clear the air.
Fair chances - What distance you were at from the teacher! The pronunciation of 'D' is like 'B' (common -ee) when heard from far. The teacher might have told 'B' that sounded like 'D' to you.