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

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

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.