Which is the most natural way to ask the question below? Are the replies correct? (Words in parentheses show that they are optional.)
Whose are these notebooks? - (Of) our students./These notebooks are of our students./These notebooks belong to our students./These are the notebooks of our students.
Whom do these notebooks belong to? - (To) our students.
To whom do these notebooks belong? - (To) our students.
Who do the notebooks belong to? - (To) our students.
As far as I know, to whom is a very formal expression. What about the rest?
What about the following question? Is to optional in the three replies?
To whom are you talking? - (To) our students.
Whom are you talking to? - (To) our students.
Who are you talking to? - (To) our students.