The whole Tragedy of Julius Caesar is written in blank verse, except for some parts of the play. Usually, the commoners speak in prose, because blank verse is used by the more educated people. However, there are a couple of instances where non-commoners speak in prose: the first person is Casca, when addressing the commoners, and the second one is Brutus.
So, the correct answer is C. Brutus: prose; Mark Antony: blank verse.