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.