"Why do you not eat fruit?"
Translation:¿Por qué no comes fruta?
We generally say "you" for second person singular (one person) AND for addressing more than one persons. No need to say "you all" as the situation usually indicates if it is one or more than one person's being addressed. So both are the same with the exception of how many persons are being addressed.
"¿Por qué?" is a phrase that means "Why?" I guess if you wanted a literal translation, "For what?" would be suitable, even though that's not technically correct ("For what" is translated to "¿Para qué?"). But, just as you have to memorize that "Lo siento" means "I'm sorry" and not (usually) "I feel it," you'll have to memorize that "¿Por qué?" means "Why?"
The "comen" form of the verb could also be used with "they", as in "Why don't they eat fruit?" The "ustedes" (you) is the subject of the sentence. The correct answer is correct in English, which suffers from an inability to distinguish between you familiar, you formal, you singular, and you plural, without adding extra explanation. Spanish (and other languages) have different words for those forms, and don't need the explanation.
Tú means you, but it is what we call the familiar you. It is used to talk to people you are close to, like family, close friends, young kids, pets, and even God. Usted (plural form ustedes) means you but it is what we call the formal you. It is used to talk to people you don't know, people above your social status, people in authority, people who are older than you are. When you first meet a person, and you want to say "you" to them, you use "usted." Eventually, if you become close friends, and you want to say "you" to them, you use "tú."