That would strike me as odd. "gibi" is making a general like-comparison here, "elma gibi" should mean like an apple. I have a hard time imagining a situation where I would talk about a fruit resembling some particular apple.
Also, mods, the generic translation here is wrong, should be "a fruit like an apple*" -A fruit like apple- actually implies to me "meyve gibi bir elma" :D
O would work okay. I THINK "bu" would be slightly closer to the common idiom but either is understood, and I'm just speaking off of feelings.
Şu wouldn't. It would seem like you're about to add something else, like you're following a train of thought to something else and proposing something about it.
(Avoid şu in general until you've observed its use in the wild. I'm not saying it's never right but şu in general has weird usages, like you're saying "this thing here" as opposed to just "this." It can be a bit pejorative or flip in the wrong context. I think it can get a second language learner in trouble by adding unintended implications.)
Aida, in "elma gibi bir meyve"="elma gibi/bir meyve" there is no verb, so no subject. We have the same construction as in "güzel bir meyve"="a beautiful fruit". "Elma gibi" is the equivalent of an adjecive. "The apple is like a fruit" would be: "elma meyve gibidir". Ok?
malak, there is no "you" in Duolingo. The course is developped by people of course, but after there is nobody who corrects us. Why a comma is wrong? Because in Turkish a comma is used to avoid confusion between a personal pronoun and a demonstrative adjective as in : "O, o çilek yer"="He eats that strawberry".