"Do you want a tea?"
Translation:¿Quieres un té?
"Usted quiere un té?" and "Quiere un té? are correct but marked incorrect while the suggested solution is "Usted quiere un café?" which is incorrect.
Edit: Same issue with the ustedes form responses.
Not in New England, at least. I never hear, "Do you want a tea?", instead, I hear "Do you want tea?" Same with coffee, or really any drink. "Do you want soda?" not "Do you want a soda?"
However, "I'll take a soda" or "I'll get a water" are used. So, in question form the "a" isn't used, but it can be used in other forms.
While it might be more polite or genteel to say "Would you like tea?", the question "Do you want tea?" IS a correct grammatical construction. It breaks no linguistic rules, has no syntactical errors, and would be understood by any native English reader.
We don't help students who speak English as a second language by trying to force prescriptive opinions on them. They are very likely to hear the construction, "Do you want X?" - in any English-speaking country.