The example of dramatic irony you provided, where Brutus underestimates Mark Antony, affects the audience primarily by heightening suspense because Mark Antony is dangerous. The audience knows that Antony is a cunning and strategic character who poses a significant threat to the conspirators after Caesar's assassination, while Brutus believes that Antony is harmless without Caesar. This creates tension as the audience anticipates the consequences of Brutus's misjudgment and its impact on the unfolding events. Therefore, the correct answer is:
It heightens suspense because Mark Antony is dangerous.