Final answer:
Brutus's reference to Marc Antony as Caesar's arm implies that Antony is a significant threat to the conspirators' cause, emphasizing his loyalty and ability to continue Caesar's work.
Step-by-step explanation:
In Shakespeare's play Julius Caesar, Brutus's reference to Marc Antony as Caesar's arm aims to highlight Antony's potential to continue Caesar's legacy and be a threat to the conspirators' plan to restore the Roman Republic. Given the historical context that Marc Antony was a loyal supporter and a skilled general who was left in command of Caesar's legions according to Caesar's will, as well as his role in the Second Triumvirate, which avenged Caesar's death, it is clear that Brutus viewed Antony as a threat due to his loyalty to Caesar and his own military prowess. Therefore, the correct answer is B) Antony is a threat to their cause.