It also lists An Nadúr, which Duolingo didn't accept a year ago, at the time that I made my comment. I have no problem adding dúlra to my vocabulary, I was just pointing out that it wouldn't necessarily be the first choice for all Irish speakers, in all contexts.
I learned the word nadur for nature when i went to school. What has changed
Nothing has changed - an nádúr still means "nature", though it's not the only word that can be used. But it is one of those words that normally takes a definite article in Irish, even though it doesn't in English. I don't know what alternative answers Duolingo accepts for this exercise, but if you get this question again, try an nádúr and use the "Report a Problem" button if it isn't accepted.