I know the idea is that we are to notice the plural, but in English we do not use fruits. We would say buy fruit.
I agree that we would say "Buy fruit." And that should, of course, be the answer of choice here. However, both fruit and fruits are plural forms of fruit. The plural form, "fruits", is more often used in a figurative sense, e.g. the "fruits of our labor", and in a literary context, e.g. "so many fruits on display" instead of "such a variety of fruit on display".