"Where is the exit?"
That usually ends up being the case:
- いちばんすきなたべものはなんがだか?（一番好きな食べ物は何だか？） "Regarding number one liked food, what is it?" / "What is your favourite food?"
- パンケーキだ。 "It's pancakes."
Here the replier can fully omit everything before and including the particle は, only replying the なん part. Same goes for any question that includes a が, you'd only reply the が part.
I really hope this clarifies anything, do ask if anything's unclear!
from what i've learn, u use 'doko' when u dont know exactly where is the place. In other side, 'dochira' used when you see options but you dont know which one is the correct one.
deguchi wa doko desu ka? ( where is the exit? )
deguchi wa dochira desu ka? ( you see some doors, and you are asking, "Which one is the exit?" )