The correct answer is "Brutus suspects that Cassius' flattery is fueled by intentions that will put him at risk, and Cassius assures Brutus that he is honest and trustworthy".
The dialog in this passage opens with a clear display of suspicion from Brutus towards Cassius. He's concerned that the latter may not be trustworthy. Cassius defends himself, explaining that he will only act as a mirror to show Brutus what he already has inside him but hasn't seen yet. Finally, Cassius reassures Brutus that even though he has many people by his side, he's not a phony, and time will prove he's not dangerous.
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