There are probably only two feasible answers at the most, but this is potentially a very subjective question. Particularly as the only acceptable answer was originally "Napa Valley".
It's just possible that someone might have heard of the Sonoma Valley (for example) but not the Napa Valley. In this case that person could, with some justification, argue that the Sonoma Valley was (in their opinion) more famous than the Napa Valley.
This question would have been better asked the other way round: "In which US state is the wine-growing area known as the Napa Valley?"
If you think that's too easy ... just ask a different question.
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