Can which be a verb?

"Which" is not a verb. It's a pronoun, specifically an interrogative pronoun, used to ask questions.

Here's an example:

* Which book did you choose?

In this sentence, "which" refers to the book, and it's asking for a specific book to be chosen.

To make this sentence have a verb, you could say:

* Which book did you choose?

Now, "choose" is the verb, indicating the action of selecting a book.