In this passage, Lucillius helps advance the plot by explaining that Cassius is far less friendly than he once was, which allows Brutus to confirm his judgment that Cassius is turning against him. By highlighting the changes in Cassius's demeanor—specifically, that he is treating Brutus with less familiarity and warmth—Lucillius provides crucial information that contributes to Brutus's growing suspicion and concern about his relationship with Cassius. This sets the stage for further conflict and tension within the ranks of the conspirators, ultimately impacting the dynamics of their alliance and the unfolding events in the play. Therefore, the correct answer is:
Lucillius explains that Cassius is far less friendly than he once was, and Brutus uses the information to confirm his judgment that Cassius is turning against him.