Il pomodoro è una frutta!
Anyone else bothered by the fact that they have the tomato as a vegetable? Not that all of the sentences make the most sense (I eat a bear...), but this is just false.
Because "frutta" refers to the food in general. "Un frutto" is the single physical object, you can't say "una frutta" (tho in some region it's used, but it's dialectal). The example doesn't fit perfectly, but frutto : frutta = coin : money. So you can say "la frutta" (the money), but not "le frutte" (the moneys) nor "una frutta" (a money). You can say "il frutto" (one coin) and "i frutti" (the coins). I hope I was clear enough
Botanically speaking quite a few vegetables are really fruits - tomatoes, eggplants, squash, peppers for example. Mushrooms are typically considered a vegetable even though botanically speaking they're not even a plant.
Duolingo's probably going by a culinary definition of fruit and vegetables - which considering that's the most likely situation that most of us will use the food words in, I don't think that's a problem.
Well, it doesn't really matter, and no one will be using Duolingo as a reference (ex. "I know the tomato is a vegetable because Duolingo says so!") so I don't think it really matters. If you really think that's incorrect you should report it as "The English sentence is nonsensical" :)
Botanically speaking, a tomato is a fruit; that is what the post was about. The same holds good for an avocado. However, all over the world these fruits are generally regarded as vegetables in cookery books and also in diet rules.