"Do you like fruit?"
Translation:Tu aimes les fruits ?
"Fruit" is also used when talking about more than one fruit (I believe it's called a collective noun?) as in fruits in general... "I need to eat more fruit". The sentence "Do you like fruit" is acceptable.
"fruit" is a mass noun that is used for "fruits" in French (plural).
"un fruit" is "a fruit" or "a piece of fruit".
Why is it not "des fruits"? The question does not ask "Do you like the fruits".
This case is an exception:
When the English sentence has an appreciation verb (e.g.: like/love/...) directly followed by a noun (no article), then the French sentence must have an appreciation verb (e.g.: aimer/adorer/...) + definite article (NOT indefinite article) + noun.
e.g.: "I like fruit." translates to "J'aime les fruits." (NOT: "J'aime des fruits.")