"What country are they from?"
It seems this language doesn't really use a different syntax when asking questions compared to making an affirmation. So, for example, 你是学生 means "You are a student", while 你是学生吗 means "Are you a student?". But not all questions require 吗 to know they're questions. Sometimes 什么, meaning "what" creates a question too, for example 你喝什么, literally meaning "You drink what" but really meaning "What do you drink?" or "What are you drinking?".
In this case I assume 哪 is just one of those particles you use to "substitute" for the very thing you're asking in the question, like a "which" or something similar. 国 means "country" and 国人 means "from X country", so, as we don't know which country you're from, we can't say, for example, 我是中国人 ("You are Chinese") or 你是美国人 ("You are American"), so we ask 你是哪国人 instead (literally "You are from which country", meaning "What country are you from?")
It's good they have these comments so I can find out more information, as to why my answer was wrong.
I kind of wish though, that this website gave a little more of an explanation and had more info I could read and study, as opposed to just repeatedly doing these "tests".
I made a mistake, folks. In my example I put 我是中国人, which is, of course, "I am Chinese", not "You are Chinese". My sincerest apologies.
Yeah, 哪国 means "which country" here. 哪 is for questions about place. I agree the presentation here is somewhat impenetrable. Check out the free program called Anki and a free deck called "SpoonFedChinese". I've only done 200 or so of those cards (out of 10000 and they keep getting more complex!) and they make everything here seem very simple, even when I don't already know the vocabulary. I think I would really have struggled to do this Duolingo course without that Anki list.
哪 In this context means "which", I don't think you're referring to the same thing... literal translation is like "they are which country's people?"
I did 他们是哪国？and it was marked wrong. Does anyone know why 人 is required here?
国 means country, and 国人 means citizen (or "country-people"). If my understanding is correct, your answer might be translated as : "What country are they?" I'm learning this for the first time, so please correct me if i'm wrong!
美國 = America and 美國人 = American. When you add the人 at the end of the county you get that's county's people. 中國= China, 中國人 = Chinese.
The last character by itself guo - 国 means country or nation, ren - 人 means person, so without the 人 you are saying which country are you? Rather than which nationality are you? (Literally: which countries person are you?) So you need the 人 to be correct