Whoever made the question doesn't understand it.
I think that's why they said, "if you had to choose." Beyond that, it made sense to me. Certainly there are both involved in everything, but if there can be foods which are more yin or yang in nature (as I've read in more than one source) then it makes sense to me that most anything else can too. Few things are perfectly in balance: expansive/contractive forces may include equal and opposite reactions, but that doesn't mean we can't meaningfully describe something as being more yin than yang or vice versa.