Is it ... To who?
Or it ... To whom?
It could be either, depending on the role of the pronoun in the sentence.
Here are all the cases for the pronouns "who / whom":
(1) "Whom Who wrote that book?" (subject: nominative only)
(2) "Whom / who did Ed meet in town?" (object of verb: accusative or nominative)
(3) "To whom / who is he talking?" (object of fronted prep: accusative only)
(4) "Whom / who is he talking to?" (object of stranded prep: accusative or nominative)
(5) "Who / whom is he?" (predicative complement: nominative only)
- when it is object of the verb, as in (2), both cases are found, but "whom" is formal; "who" is preferred in conversational English by most people.
- when the preposition is stranded, as in (4), "who" is strongly favoured.