Question

The pronoun whom is which type of pronoun? A. Possessive pronoun B. Subject pronoun C. Personal pronoun D. Object pronoun

Answer 1

In fact, they apply only to personal pronouns, possessive pronouns, and reflexive pronouns. It is only in these types, too, that gender differences are shown (personal he/she, possessive his/hers, reflexive himself/herself). ... Many of the pronouns listed above also belong to another word class - the class of determiners.
Pronouns @ The Internet Grammar of English - UCL

Answer 2

The word "whom" would be a personal pronoun becuase it is talking about a specific person just try to ask who or whom did that.