It should be "which," I believe. Webster defines "which" in this way:
1 : being what one or ones out of a group -- used as an interrogative
It defines "what" like this:
1 a (1) -- used as an interrogative expressing inquiry about the identity, nature, or value of an object or matter (2) -- often used to ask for repetition of an utterance or part of an utterance
Sorry about that. I copied and pasted from M-W's online site, and I didn't bother to get rid of all the extra stuff from the definitions. My apologies!