English question, please help?

plural is correct

If you are talking about a flavour then apple, 
if you are talking about the fruit itself then apples.

If you're referring to apples as a type of fruit, apples would apply, but, if you mean the particular apple you're eating, then one might say, "I like this apple."

both are correct, but in different ways.  I like apple (the taste of apple or its smell, or something that is "apple" without meaning the physical apples involved) is often used for the general idea of apple as a food.  I like apples is used to mean the things themselves, not just a basic character of them, but the actual real apples.

I like apples is correct

The second one is correct.

Either could be correct.
“Do you like apple (adjective) pie, or chocolate pie?”
“I like apple (pie).”

“Do you like apples (noun), or oranges?”
“I like apples.”

“What do you like for a bedtime snack - an apple (singular noun) or a banana?”
“I like an apple.”

I like apples. 
I like the flavor of apple. 

Meats become noncount; some vegetables do; fruits don't .

I like chicken.
I like beef.
I like corn.
I like asparagus.

BUT I like peas and beans and potatoes.

I like strawberries, plums and peaches.