Had this too
Answer B.They murdered Caesar for “justice’ sake,” but now Cassius acts unjustly by taking bribes.
You find your answer by the very last line: shall we now
Contaminate our fingers with base bribes . . . ?
Even if you don't know completely what is going on with Caesar, that is fine. As long as you see that the character is feeling contaminated - dirty from the idea of bribe that is all you need know.