What and which are both interrogative adjectives that you can use to ask questions, but how do you know which one to use? Let's get all meta and examine that question itself.
In the previously stated question, we use which to modify one because we know the range of choices available to use: our options are which or what. If you don't know what options are available, then it's correct to use what.
Easy as that.
Example
What Beatles album is your favorite?
OR
What song do you think the Rolling Stones will do for their encore tonight?
The Beatles and The Stones: a rivalry as old as rock 'n' roll itself. Okay, so it's not literally that old, but it is almost as old as Keith Richards, and that guy is ancient. Or possibly dead by the time you're reading this.
But which sentence uses the interrogative adjective what correctly? It's the second one—because you don't know all of the options that are available. It's likely that the Stones will play something from their own catalog, but you never know. Maybe they'll throw down with some Taylor Swift. In the first sentence, you should use which instead of what because there's a definite number of sublime Beatles albums from which to choose.