Answer: who
Explanation: "A relative pronoun is a word that introduces a dependent (or relative) clause and connects it to an independent clause. A clause beginning with a relative pronoun is poised to answer questions such as Which one? How many? or What kind? Who, whom, what, which, and that are all relative pronouns.
Relative clauses are also sometimes referred to as adjective clauses, because they identify or give us additional information about the subject of the independent clause they relate to. Like adjectives, these clauses in some way describe that subject. Relative pronouns, like conjunctions, are words that join clauses—in this case, a relative clause to its main clause. The type of relative pronoun used depends on what kind of noun is being described." Catherine (Traffis, Grammarly)