When followed by a verb at the start of a question, "who", "which" and "what" are pronouns, specifically interrogative pronouns (e.g, "Who are you?", "Which do you want?", "What did you say?"). In the same circumstances, "how" is an adverb (e.g. "How do you feel?").
When followed by a noun at the start of a question, "which" and "what" are determiners, also called adjectives (e.g. "Whic