My uncle was not sure ___________ the strange guest were A) which B) who C) whom D) what

Answer 1

It is whom. A simple way that works most or the time for determining if it is "who" or "whom" is if you can replace "he" or "she" with the "who" and the sentence still makes sense, then use "who". However, this fails with this sentence. And "which" and "what" are normally used to refer to objects. Therefore, whom is correct.