Final answer:
The pronoun 'who' in the sentence is a subjective case pronoun as it acts as the subject of the relative clause.
Step-by-step explanation:
In the sentence 'Marcia is the girl who lives upstairs,' the italicized pronoun 'who' is a subjective case pronoun. This is because it serves as the subject of the relative clause 'who lives upstairs.' Subjective case pronouns function as the doer of the action or as the subject in a sentence or clause. Examples of subjective case pronouns include I, we, you, he, she, it, they, and who/whoever. In contrast, objective case pronouns function as objects, which include pronouns like me, us, you, him, her, it, them, and whom/whomever. Possessive case pronouns show ownership and include forms like my/mine, our/ours, your/yours, his/her/hers, its, their/theirs, and whose.