Hello. :) Well, I wouldn't use any of those if I were to talk about my favorite fruit. Instead, I would say "My favorite fruit is apple." I'll explain why each one sounds "weird" to me. Be prepared to read a lot! ;) XD **First, I would say, "My favorite fruit is apple." because a lot of times we think of only one apple when we think of that fruit, right? (Would you really think of a whole bag of apples?) However, when we think of grapes we think of a lot instead of only one grape, right? Therefore, it would be okay to say, "My favorite fruit is grapeS." (1)My favorite fruit is apples. The verb is "is" (singular), therefore, the noun/object cannot be plural. *More could be explained here, but I will not do further "discussions" because there's a limit I can write (on here). (2)My favorite fruit is the apple. "..the apple." means that there is only ONE (and ONLY ONE) very particular apple; you eat it, and that's it; no more of that kind. However, this sentence could be extended longer if you still want to use "the" in front of "apple." For example, you could give more information about the apple by saying, "My favorite fruit is the apple...on my desk."-->But this would be weird if you said this, because (similar to what I mentioned above) it's as if you're talking about only ONE very particular/special apple. Anyway, another way to say it, if you still want to use "the apple", is to describe it. For example, "My favorite fruit ARE the GREEN appleS." *If you're wondering why I said "fruit" instead of "fruitS", then it's because if I said "fruitS", then it's like I want to talk about other types of fruits. However, since I'm only talking about one type of fruit, which is apple, then it's okay to say "My favorite fruit are the green apples." (3) My favorite food is an apple. This is similar to #1. An=one. **Great question though! This is actually a harder topic that native speakers still tend to struggle to explain sometimes. Sorry; my limit is up! :/

Agree with Liya.

Number 2 is correct. Number 1 is also correct, even though it may strike some people as problematic. Number 3 is not English, unless you're trying to say that there's one apple that you like more than any other apples and more than any other pieces of fruit.

My favourite food is an apple