I'm an educated native English speaker and would say that, but DL doesn't accept it. "What" is used to ask about something with an indefinite number of responses, whereas "which" is generally used when there is a discrete number of choices. In real life either would be OK in that sentence. Depending on the situation in which the question is asked, either could be technically correct as well. In this sentence, because there is no obvious discrete number of choices, "what" is actually more correct. I'm flagging it.