I'm may not be that good at math, but I'm going to try my best to help you, because I know how it feels to be in this situation!
The first one :
You might be right on D. But I'm thinking B also. The question asks "Which could be a cross section of a rectangular pyramid that has been intersected by a plane perpendicular". But a triangle is basically a flat pyramid.
(I'm typing really fast to help!!!!)
Second one: Its B!! Look at it really closely! You'll get it!!!
(Going really fast now!!!!)
Third one: Between B or D!!! But I think you're right on your choice!!!!
(FASTER FASTER!!!)
FINALLY: Copy and paste your answers into google. People have asked the same question!
I hope I helped. I don't want to make you fail, as I am in the SAME similar situation as you! I really REALLY REALLY HOPE YOU PASS!!!