The fifth and final act (1)/ of Macbeth contain (2)/ the sleep walking scene. (3)7 No error. (4)

Answer

2

Explanation

contains will replace contain because - The fifth and final act (Main Subject) is in Singular. Hence, the Verb will also be in Singular as a Verb agrees with its Subject in Number. Singular Subject `rarr` Singular Verb (Main Verb +S) Plural Subject `rarr` Plural Verb (Main Verb without S)
Look at the example given below:
The great poet and scholar is dead. Here, scholar and poet refer to the same man. So, the Subject is in Singular and the Verb is also in Singular. Hence, of Macbeth contains is the right usage.