Answer: B
The explicit meaning in this passage is best expressed by the alternative B: "Brutus told you (Brutus hath told you ) Caesar was ambitious. If it's true that Caesar was (if it were so), then it wasn't a good thing (it was a grievous fault ), and he's been punished for it (and grievously hath Caesar answer'd it )."
Grievous: having very serious effects or causing great pain.