Answer:
It was the cut that finally killed Caesar and because it was from Brutus, whom Caesar considered a trusted friend.
Step-by-step explanation:
First choice: there is no proof that Brutus stabbed Caesar first, nor that he stabbed Caesar in the back
Second choice: there is no proof that it was the most gruesome cut
Fourth choice: Brutus was not a known enemy of Caesar's; in fact, he was Caesar's friend
The third choice is the only one left (and there is information to prove this), so the third is correct.