Read the sentence:

the pea green sofa that used to sit in my college dorm room is in my office now.

which is the relative pronoun, and what is its function in the sentence?

question 13 options:

a. the relative pronoun is that; it functions as part of the adjective clause.

b. the relative pronoun is that; it functions as part of the adverb clause.

c. the relative pronoun is in; it functions as part of the adjective clause.

d. the relative pronoun is in; it functions as part of the adverb clause.

The correct answer is option A.

In the sentence "the pea green sofa that used to sit in my college dorm room is in my office now" the relative pronoun is "THAT", and it functions as part of the relative/adjectival clause "that used to sit in my college dorm room".

In addition, the adjectival clause is introduced by the relative pronoun "that" which has the syntactic function of subject, since it is followed by a verb.  

The correct answer is A. the relative pronoun is that; it functions as part of the adjective clause.

A relative pronoun is a pronoun used to connect a clause to a noun or a pronoun. In this case, the clause is "that used to sit in my college dorm" and the pronoun is "that" because it is referring back to the pea green sofa. Moreover, "that used to sit in my college dorm" is an adjective clause since it is a dependent clause that is describing the noun "sofa". "In" could not be a relative pronoun because it is a preposition.