"Who is the mother?"
Translation:Quis est mater?
Quis mater est as an answer is not incorrect; Latinists traditionally put the verb as the last word in a sentence. Vita bona est! 28 August 2019
It should also be accepted now, but unfortunately it takes some time for the changes we make in the Incubator to be active for users (sometimes as long as two weeks).
Still, please report (with the button in the lesson, not in the discussion) if it's not accepted, it's still possible something got missed!
But isn't it true that verb placement is often changed from the declarative to the interrogative?
It certainly can be. Whether it's done often I don't know, I'm actually not a Latin expert, just temporary help for handling the initial flood of reports on the course. :)