Cá is a simple interrogative and will always need a verb. You may be getting it confused with cé which does imply the assertive verb (is) and would take the disjunctive pronoun or a relative clause (using that pesky a again)
Cé tú? - Who are you?
Cé hí? - Who is she? (Cé causes h-prosthesis with é, í and iad.)
Cé a rithfidh inniu? - Who will run today?
Cén (What, which) is cé + an and would be followed by a singular noun--I think that's what you're thinking about. (For plural noun you would use cé na.)
This is useful and ended up (all of it) in my notes.
I was actually wondering why it wasn't worded like Cá an bhfuil tú? which was my initial take on this one.
Maybe I am trying a tad too early to converge duolingo Questions content with Learning Irish content to make some sense :)
edit: for example, I learned An bhfuil tú anseo? (Are you here?), so I thought "Where are you?" would have the same verb form with the extra cá
An is called an interrogative particle and it's function here is simply to introduce the verb when you are not using a specific interrogative in the question. So when you use cá it simply replaces that particle. That's all.
But note--and this is where these little words will drive you a bit crazy-- in Cén, the an that is in there is not the interrogative particle an but the definite article an. That is why cén works differently here.