An occurrence of the word which.
"The question remains, which answer is correct?"
"The police officer pulled his gun and pointed which way he wanted us to go."
What, of those mentioned or implied.
"Which of these books did you borrow from the library?"
What one or ones (of those mentioned or implied).
"I have three books, and which one do you want to borrow?"
The word "which" comes from Old English, originally meaning "what kind." Its roots trace back to Proto-Germanic, where it literally meant "like what."