Interrogative pronouns are used to ask questions.

• who: as a subject/an object, as a pronoun
• which: as a subject/an object, as a pronoun/determiner
• whom: as an object(formal), as a pronoun
• what: as a subject/an object, as a pronoun/determiner
• whose: as a subject/an object, as a pronoun/determiner

http://dictionary.cambridge.org/grammar/british-grammar/pronouns/questions-interrogative-pronouns-what-who https://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/interrogative-determiners-which-and-what