"Whose daughter is she?"
Translation:Čí je to dcera?
It is not impossible, but it does really scream that you just translated the English words in the English word order.
The order you propose strongly stresses ONA. Whose daughter is this one and not that one?
I will add it though as we cant teach here the correct Czech word order anyway.
Ah, but the instrumental case actually is NOT required, as we can see by the fact that the main translation -- the one at the top of the discussion page -- uses the genitive, which has been introduced.
DL sometimes offers weird "corrections" when we give an answer that's so different from the main translation that it thinks we're trying to use something more... I don't know... sophisticated.
So I'm guessing that's what happened to the OP in the exercise... and it's happened to me plenty of times, too!