Correct Answer: Pronoun

Solution : The correct answer is option 2.

Explanation:

The pronoun "who" is underlined and is used to start a clause that provides more details about the person or people who were previously mentioned.

For instance, She is the one who took his book.

"Who" is a relative pronoun.

A relative pronoun is a word that joins an independent clause to a dependent clause.

In order to give the subject (noun or pronoun) it refers to more information, relative pronouns are used.

These relative pronouns can also be referred to as adjective clauses because they behave exactly like adjectives.