The Rule for When to Use "Who" and When to Use

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