Answer:
A. a subject
This sentence already has a predicate, pretty much the whole sentence is one.
It doesn't need an adjective (a describing word) to complete the sentence.
It doesn't need a conjunction (is, and, but) to complete the sentence either.
What we need in this sentence is a subject, or a noun. What is his favorite movie?
Complete Sentence:
My brother's favorite movie of all time is (subject).
