Answer:
In his great funeral oration in Act III of Shakespeare's Julius Caesar, Mark Antony
masterfully manipulates the crowd, turning them against Brutus and Cassius.
Step-by-step explanation:
According to the given sentence, the grammatical error there was the improper use of the adjective "masterful" to describe how Mark Antony manipulated the crowd.
However, this is corrected by replacing the adjective "masterful" with the adverb "masterfully".