The rule for when to use “who” and when to use “whom” is that

• A. “whom” refers to the subject of a clause and “who” refers to the object of a verb or preposition
• B. “who” is acceptable in spoken language but “whom” is preferred in written language
• C. “whom” is used only in scholarly writing and “who” is used in more informal writing
• D. “who” refers to the subject of a clause and “whom” refers to the object of a verb or preposition