his external conflict with Cassius This is the correct option.
Brutus has the feeling his bond with Cassius is becoming weak. Brutus has this conversation with Lucillus before he and Cassius decide to have a private conversation. Brutus and Cassius had an argument about corruption, yet they make peace during their private meeting. They remain friends.
These options are nort right.
-his external conflict with Mark Antony. ( Mark Anthony is Brutus rival. He does not doubt about it).
-his internal conflict over killing Caesar. ( He has this conflict but it is not reflected in these lines).
-his internal conflict about the state of Rome. ( Same as above).