Translation:Which desk is it?
What I want to know is why Duolingo keeps asking me which desk, or which chair, or how much these chairs and desks cost in the section titled Restaurant. I'm not going to a restaurant to eat the chairs! It makes it seem like there's a cost to simply sit down in a cafeteria.
Well, because these are the few words we've learned so far. However, you can simply research a word and use in this sentence structure. It's always good to learn some structure, don't you think so?
Anyway, in another place, like in a joinery or in the furniture section of a mall, it will be a useful sentence.
Not quite. Dono is part of the same set of demonstratives as kono, sono, and ano. They are adjectives, and must be followed by a noun, eg どの椅子, or この椅子. So you will never see "どのですか"; only ever "どの x ですか", which would translate to "which x is it?"
Dore is part of the same set as kore, sore, and are. These are pronouns; they completely replace a noun, and hence can stand alone. So you can say "どれですか?" which simply means "which is it?"