"Il boit un café et de l'eau."
Translation:He drinks a coffee and water.
The point of this exercise is to show the difference between countable and uncountable things. In French and in English it is possible (and in some places quite common) to shorten "a cup of coffee" to "a coffee" / "un café". He is drinking a coffee. (one cup) Il boit un café. Water, on the other hand, is never shortened to "une eau" in French. It is uncountable.
This nuance is lost if it is written, "il boit du café et de l'eau / he drinks coffee and water". How much coffee? We don't know. The meaning is different.
In the U.S. one would rarely if ever say "a coffee." It is coffee or some coffee, or a cup of coffee. However, I know "a coffee" is standard in Ireland. Possibly in the British Isles as well. I do not know about other English speaking countries such as Australia, Canada, New Zealand, etc. So Duo should probably accept "Do you want 'coffee,' or 'a coffee,' or 'some coffee.'"
I hear my co-workers (in the U.S.) say all the time that they are going to get "a coffee" at the Starbucks downstairs, inviting their fellow coffee enthusiasts to join them. They do not say "a coffee" in relation to coffee from the break room. That would be "some coffee" or just "coffee."
I believe that paying $5 for a cup of coffee gives it special status as "a coffee." :)
I agree that all three should be accepted.
(I don't drink coffee, no matter how it is referred to.)