"The girls' newspaper"
How could I know the answer if I didn't studied it earlier? I know they book is leur livre, but nobody mentioned earlier the girls' book is le livre des filles :S Maybe the system should tell some grammar rules before the practice and taking away hearts.
That is not the way Duo works for the time being. Duo teaches you words and structures with examples. You may fail the first time you translate them, but failing is also learning. If you need explanations or clarifications, you have to ask for them and another learner will help you.
If you would specify what your issue is with that sentence (the English for? the French form?), I will be pleased to answer.
Note: "jeune fille" means "young woman" (unmarried teenager or young adult).
to mean "young girl", we use "petite fille" (@4-12 years old)