I initially read it as "The girls. Are they poor?" and took the first half to just be Duo giving us some context. As an above comment mentioned, when forming questions there seems to be three ways to go about it from most formal/correct to least:<pre>
Est-ce que les filles sont pauvres? Les filles, sont-elles pauvres? Les filles sont pauvres?</pre>
The very last one, as far as I can tell, is used only orally and is highly informal, possibly even heading into slang. The top two are both correct with the top one being the most formal; either of these top ones are ok for writing letters, etc.
The top one is the classic way to form a question. The middle one is an alternative way to form a question and requires inverting the verb-pronoun phrase. The above comment mentions that a pronoun must be used, not a noun. It may be redundant, but every language has it's odd points.
Just a small correction here, if you don't mind. Inversion verb-subject (or verb-subject pronoun, as here) is always the most formal form of asking a question. Using "Gallicism" (as one of the users here called it on several occasions) "est-ce que" followed by a regular order of subject, verb and object in a sentence is a standard, usual way of asking a question, but it is always a less formal one then the inversion verb-subject form.
You've been waiting a month, so I'll try. Maybe someone more fluent can explain it better later. :) If you use the question form Est-ce que... then you can just leave the words in the usual order and you don't need the extra pronoun 'elles'. (Est-ce que les filles sont pauvres?). However, if you use the inversion form of the question, you have to use the pronoun (elles) after the verb (sont). If you were simply asking "Are they poor?", you wouldn't need the extra subject 'les filles'. Because this example is asking "Are the girls poor?", you have to have 'les filles' in there, but you also need the pronoun 'elles' because of the inversion 'sont-elles'. I think. :) (Sorry this is so long!)
Thank you for casually blowing my mind with this comment. It was unexpected and enlightening at the bottom of the page here :) I had always thought 'si' to mean 'if'. Because of you, I went researching this and learned more about the french language. Thank you for simply writing, 'I hope so'! :)
I can only say that the few times I've seen this construction outside of Duo, there's no comma. In any case, I don't think this actually qualifies as a case of emphasis (as you might see with the tonic accent). Rather, it's a case of necessity if you want to use inversion, since you need to preserve the "les" somehow. If I'm right about that, then you wouldn't expect a comma.
Multiple choice error number 59. 'Pauvres' left out of the sentence but not offered as a possible answer. I do not think I am ever going to be able to complete this section on adjectives if these errors aren't corrected. PLEASE, Duolingo, check ALL of your multiple choice questions!