Is which a noun or pronoun?

"Which" can be both a pronoun and an adjective, depending on its role in the sentence.

Pronoun: When "which" stands in for a noun, it's a pronoun.

* Example: "I have two cars. Which one do you want to borrow?" (Here, "which" refers to the cars, replacing the noun phrase "one of the cars.")

Adjective: When "which" modifies a noun, it's an adjective.

* Example: "Which book did you choose?" (Here, "which" modifies the noun "book.")

So, to determine whether "which" is a noun or a pronoun, look at how it's used in the sentence.