What part of speech that answers question which one?

The part of speech that answers the question "which one?" is a pronoun.

Here's why:

* Pronouns stand in for nouns.

* Interrogative pronouns are used to ask questions.

* "Which one" is a phrase that functions as an interrogative pronoun, asking for a specific selection from a group.

Examples:

* Which one did you choose? (Which one = pronoun, asking about a specific choice)

* Who is coming to the party? (Who = pronoun, asking about a person)

* What is your favorite color? (What = pronoun, asking about a thing)