Don't worry. You can't be wrong. Apple is a fruit. Apples are a fruit. Whatever you say, I can devise a grammatical justification for it. That's because the noun "apple" can name several different things. It can name a single thing that you can hold in your hand. It can name a species, which is a sort of collective plural. So just say whatever you like and you'll be all right.

My favorite vegetable is a tomato. My favorite fruit is an apple. My favorite vegetables are tomatoes and potatoes. My favourite fruits are apples and pears,

I'm not sure if this is a rule, but this is my opinion. For things like this you can use either: Singular with an article OR Plural with no article Ex: My favorite fruit is the apple. My favorite fruit is apples. BUT NOT My favorite fruit is apple. BUT YOU CAN HAVE My favorite flavor is apple. That's my opinion anyway.