Can the word which be a pronoun?

Yes, the Word 'which' is a pronoun and an adjective.

The Word 'which' is an interrogativepronoun as a Word that introduces a a question.

example: Which do you like best?

The Word 'which' is a relative pronoun as a Word that introduces a relative clause.

example: I don't know which I like best.

The Word 'which' is an adjective when placed before a noun to describe that noun.

example: He finally decided which one he would buy.