The detail that supports the belief that Cassius is dishonest and self-serving is that he forges letters by the citizens and plants them for Brutus to find. Option A is correct.
Cassius is so manipulative that uses Brutus's good nature and character in his attempt to destroy the life of Julius Caesar. Cassius writes letters, claiming they are from the people of Rome, and sends them to Brutus to make Brutus believe the citizens of Rome are worried about Caesar's power.
