Question

Then yield thee, coward, And live to be the show and gaze o' th' time!

Answer 1

This is a taunt. Here MacDuff is telling MacBeth to yield and show himself to be a coward as well as a monster. People will come to see you, and you will be painted on a pole and underneath will be written "Here may you see the tyrant."

Act 5 Scene 8.
This is one of those multiple guess things that the correct answer is in the mind of  the asker. You could argue all of them to be true. He does blame Lady MacBeth who went mad bearing that blame.

He (hypocritically) befriends Duncan even offering him protection and hospitality, only to betray both and this betrayal is certainly one of  the worst possible crimes.

He murders the king. 

And he does refuse to listen to the witches which are practically his only true friends throughout the play. They say what he needs to hear.

I see why this question is so hard. There is no answer that is easily eliminated.  

His most villainous act is the betrayal of his lord and master, a king that ruled over him. I see subtle implications in B which likely is not what your teacher would choose.

C is probably what you are looking for, although I'm not sure anyone in the play has full knowledge of what he has done. Most would believe the concocted version that the guards were responsible.

Be prepared to get this marked wrong, but I would argue it politely and firmly, because all the choices are possible.