What's about: The grapes are very sour? It was marked wrong. I can't remember, that Hungarians use the plural for grapes. But in English, German or French there is a difference between singular and plural for grape/s, Traube/n, raisin/s I think, both should be accepted. What do you mean?
Szőlő can mean an individual piece of grapes or can be used as a mass noun. Szőlők is either several individual pieces of grapes or several kinds of grapes. I personally wouldn't use the plural if there are more than a few handfuls of grapes, especially if they're in the same bunch (fürt.)
With that said, the correct translatiion would be "(the) grapes are very sour."