"Who climbed up that mountain?"
Duolingo prompts you to use その but I'd assume あの should be accepted. Sometimes kanji substitution gives me アの instead, which I'd think is wrong. Anyway, I am amused by the concept of the speaker being away from the mountain but somehow talking to someone near the summit of the mountain, justifying using その instead of あの
I wrote その山に登ったの人が誰ですか？ I think I can already see some issues with this but need help identifying grammar errors or determining if this is even a valid sentence structure. I feel like I've seen this structure in other sections and it was the first one that popped into my head for some reason.
That's "Who's the person that climbed up that mountain" - the meaning is essentially the same as it should arguably just be accepted, but just as there are some subtle differences in English between when you're more likely to use one form vs the other, presumably there are between the two different Japanese structures too.
(One slightly far-fetched but at least clear example - in a fictional/future world where you could ask such a question of an animal or a robot, you wouldn't ask "who's the person"!).