An interrogative adjective ("which" or "what") is like an interrogative pronoun, except that it modifies a noun or noun phrase rather than standing on its own (see also demonstrative adjectives and possessive adjectives):

Which plants should be watered twice a week?

Like other adjectives, "which" can be used to modify a noun or a noun phrase. In this example, "which" modifies "plants" and the noun phrase "which plants" is the subject of the compound verb "should be watered":

What book are you reading?

In this sentence, "what" modifies "book" and the noun phrase "what book" is the direct object of the compound verb "are reading."