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'.