Answer: The correct answers are:
Whom is used as an object.
Who is used as a subject.
Explanation: In a relative clause, "who" is used as the subject of the verb, while "whom" is used as the object of the verb or preposition. To determine whether to use "who" or "whom," you should consider whether the pronoun is acting as the subject or object of the verb or preposition in the relative clause.
For example, in the sentence "The woman who(m) I met yesterday was very nice," "who" or "whom" is used to refer to the woman in the relative clause. In this case, "who" is correct because the woman is the subject of the verb "met." If the relative clause were "I gave the flowers to the woman who(m) I met yesterday," then "whom" would be correct because the woman is the object of the preposition "to."