Hmmm. Thanks. I didn't really think about "Where?" as being a complete sentence in itself in that way.
Cá bhfuil ?. Where is ?. Cén áit? Where? (Literally Which place?) Cén áit can stand alone.
I get caught out by this one. Why does Cá bhfuil require a subject? Also, lots of other questions aren't complete sentences, so it isn't obvious that this one should be.
I'm with you on this one! Given Duolingo's other practice examples, it's not clear to me that only "cén áit" would work as an answer here.
Much like if someone says Where is....you would expect more. Cá bhfuil sé? Where is it? Cá bhfuil Seán? Where is Sean? But if someone said There's a party tonight. You might simply say Where? Cén áit? Or Where is it? Cá bhfuil sé?
I think it would be unusual to hear it on it's own.
You might hear Cé? on it's own for "Who?", but you won't usually find Cad? for "What?", it will be Cad é? or Céard?