Subject is the function of the relative pronoun in the adjective clause.
A relative pronoun is used to connect a clause or phrase to a noun or pronoun. You see them used everyday with the most common relative pronouns being: who, whom, which, whoever, whomever, whichever, and that.
The correct answer between all the choices given is the second choice or letter B. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.