“What” and “which” are two interrogative pronouns that are frequently confused. Below is an explanation of the different uses of these pronouns.
“What” is used to ask a question when there is an unknown or infinite number of possible answers or options. It is used for asking questions in general.
In contrast to “what”, we use “which” when the possible answers are limited to a small quantity (perhaps 2, 3 or 4) and it is necessary to choose among them. This interrogative pronoun can be used with both things and people.