Three Idiots . . . . . really a watchable movie.

Answer: Option B

This question tests your understanding of subject-verb agreement. The subject of the sentence is "Three Idiots," which is a singular noun phrase even though it refers to multiple people. Think of it as a single title referring to one movie.
Therefore, we need a singular verb to agree with the singular subject.
Option A ("are") is a plural verb.
Option B ("is") is a singular verb.
Option C ("superb") is an adjective, not a verb.
Option D ("do") is a plural verb.
The correct verb to use is "is" because it agrees with the singular subject "Three Idiots." The complete sentence is: "Three Idiots is really a watchable movie."