Answer: I would contend that the right answer is the B) They murdered Caesar for “justice’ sake,” but now Cassius acts unjustly by taking bribes.
Explanation: Just to elaborate a little bit on the answer, it can be added that in this excerpt Brutus is reminding Cassius of the reason why they killed Caesar during the Ides of March. Caesar, he says, bled "for justice' sake," since he had been accused of being corrupt. Brutus brings this to the attention of Cassius because he is accusing him of being corrupt as well, hence the question that he poses to him: "[...] shall we now contaminate our fingers with base bribes, and sell the mighty space of our large honors for so much trash as may be grasped thus?"