"Favorite fruits are grapes" or "favorite fruit is grapes" [duplicate]

I think I understood the general idea of fruits vs fruit, but what about the situation where you're saying: "My favorite fruits are grape"?

This made me cringe when I saw it in an English course booklet. Would I be correct to assume that, in this case, we would say: "My favorite fruit is grapes"? The reason being that my favorite fruit is. But also, grapes is considered one group or type of fruit here, therefore fruit = is grapes.

Did I explain this correctly? Is this the right assumption? I think that "favorite fruits are..." would be used if you're going to say "grapes and apples".

Solution 1:

"My favorite fruit is grapes." As you're talking about one (kind of) FRUIT and it's singular. Just like other things, you don't say "My favorite subjects are science" what you say is "My favorite subject is Science"