That was my answer as well, also marked wrong. In my opinion it is not wrong, I learned the word Взрослый by reading The Little Prince in Russian, where the English translation uses the word grown-ups the Russian uses взрослые.
Not any more; the opportunity to add an explanation or comment to one's error report has disappeared. – a disastrous change, and one which I fear is beyond the control of our course-writers, since it seems to prevail throughout Duolingo.
The word "adult" (the person)/"grown-up" is взрослый - it is a noun but is treated like an adjective for purposes of declension. Another example would be животное (animal), or контрольная (test, even though you can also say контрольная работа).
Yes. I didn't recognise the word so typed what I heard, including 'зи'. I suppose that's the same non-vowel as in м-не and D-митри (and Edm-nd…).
And listening again at full speed, the audio seems to add a different extra syllable: 'для вз-роРОслых'.
The word adult basically serves as an adjective to describe novel. Even though "novels" is plural, the word modifying it in English will still be singular. Otherwise it sounds possessive (the novels of an adult), even though you don't have the apostrophe.