Translation:Who is this person?
Not exactly: the former means "who is this?" and the latter means "who is this person?" because the latter indicates "人". In some contexts, e.g., when the question is clearly asking about a person, yes, those questions are the same, but, if a child is introducing you to a pet cat or a puppet for example, the former question would be more appropriate than the latter.
If the DuoLingo app does not accept the former, maybe it's because one of the points of this lesson is to understand how 个 and 人 function.
No. Your phrase may imply the "who", however it doesn't work that in Chinese. 《什么人是这儿的？》doesn't make sense. You'd have to stick to the subject+verb+object order and use 谁 (when inquiring about a person)
Could you use this sentence in a situation like, if you didn't recognize someone's phone number when they call you?
Why is "[这个]人是谁?" wrong that is what I have 这个 but I think it is asking for 这 and 个 separate. Which are also selectable.
There's too much ambiguity with "guy", because in some English, it only refers to men. 人 indicates a person, but not a gender. 男人 would be "man", and 女人 would be "woman". There's a lot of slang for dude/guy like 老铁, 哥们儿, 兄弟, but it's best to learn things properly before using slang.
What's the difference between saying:"who is this person?" "And who are this persons?" ?
这个人是谁 its for both?
I believe 这个人是谁？ could be used for either "Who is this person?" or for "Who are these people?" (or for "Who are these persons?"); however, you could use 人们 to specify "people" (plural) rather than person (singular): 这个人们是谁？
In modern English, there is sometimes a subtle or technical distinction between "persons" and "people" (generally, we say "people" rather than "persons" unless we are specifically drawing the distinction or using "persons" in its technical, e.g., legal, context). I do not know how or even whether Chinese makes this same distinction; so, I am sorry I cannot help with specifying "persons" as opposed to the general "people" in Chinese.
"who are this persons" is not correct English, so nobody ever says it like that. You would need to say "who are these people" and then I think your translation would be correct.
Historically, it was commonly held that "persons" was the plural of "person", while "people" was a generic plural. So, you could say "all people are welcome" or "we invited six persons".
Usage has changed in the past century, and while legal convention is conservative in its use of these words, most style guides today prefer the word "people" over "persons" always.
In short, "persons" is correct, but quickly becoming archaic usage.
There are a couple of cases perhaps worth mentioning where these words really aren't interchangeable:
As a collective noun, always use "people" (e.g., " the Chinese people").
When using "person" to refer to a body (e.g., "he had it on his person"), "persons" is the preferred plural.
not really. 《这 》refers to "this" while 《那》 means "that". there's a difference in the subject's relative location.