This is the boy 'who did it'.The underlined clause is ——

The correct answer is:

c) Adjective clause

In the sentence, "This is the boy who did it," the underlined clause "who did it" is an adjective clause because it provides additional information about the noun "boy." Adjective clauses, also known as relative clauses, function to modify nouns. In this case, the adjective clause "who did it" describes or identifies which specific boy is being referred to.

An adjective clause is used to add details or qualities to a noun, and it is introduced by a relative pronoun (in this case, "who"). Option (c) is the correct classification for the underlined clause in this sentence.