what does mean "of whom are you speaking?"
here is the question..
_____ are you speaking? I didn't catch the name.
1.Who
2.Whom
3.Of who
4.Of whom
why the question is not"who are you speaking of?"what about if I wanted to ask"who are you speaking to?",so can I say"To whom are you speaking?"

Question

Answer

Who are you speaking of? = Of whom are you speaking?

You can write such question either way. They mean the same.

"Who are you speaking to?" = "To whom are you speaking?"

Answer

English grammar enthusiasts don't want to end a sentence with a preposition (words such as of, to, or by), so "Who are you speaking of?" is properly written as "Of whom are you speaking?"

"Who" becomes "whom" because it's the object of the preposition "of".

You are also correct in saying "To whom are you speaking?"