The right answer would be A. Conceive.
The verb to conceive means to form an idea, to imagine something, to think about it. So when we think about Sam as being an honest person, it is implied that he doesn't cheat.
Sam is so honest hat he could not even conceive of cheating on a test even if his teacher stepped out of the classroom.= Sam is so honest that he could not even think about cheating on a test, even if his teacher stepped out of the classroom.