Αντίο is not a word of greek origin, but a loanword, as stated in many comments above. So its pronunciation should be without a "n" sound. Most Greeks pronounce "ντ" like a d anyway, even in greek words nowadays. There's no point denying the truth - the language is a living organism and constantly changing.
YPSILONZ, you are absolutely right, "Αντίο" comes from Italian "adio" and therefore Greeks should pronounce it like it is written with the "ντ" sounding like soft "d" in English. I have listened to the male bot pronouncing always as "nd" which apparently is accepted by Greeks.
Most likely. Here is what I found: http://www.etymonline.com/index.php?allowed_in_frame=0search=adios: "... from Spanish adios, from the phrase "a dios vos acomiendo "I commend you to God;" the French form is adieu."
See Stergi3's post above for more.
But you didn't Report it on the exercise page so we cannot see what you wrote. We don't even know what kind of exercise it was.
Here's how to report.
Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error.
-The "Correct solution" is unnatural or has an error.
-My answer should be accepted. This is the one you should click on if your translation was not accepted
HOW TO GET THE GREEK KEYBOARD, ADD ACCENTS AND OTHER HELPFUL INFORMATION
Now here are some links and tips to help you out.
These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it.
It will also help you learn the alphabet and where to find other useful links.
And here is another to help you navigate Duolingo
FAQ - General Questions, Bugs & Reports
How to add the accent:
On the Greek keyboard first type the key on the right of the Λ (L) (the semicolon :/;) then type the letter you want the accent on.
On a mobile just hold the letter for an extra second or two and there will appear a variety of that letter with accents. Slide up to the one you want. Some simple rules to get you started:
1st rule of Duolingo: read the comments before posting.
2nd rule; read the Tips & notes on the page with the lessons
3.rd rule: use the hover drop down hints....move your cursor over a word and the meaning will appear.
And finally, Duo will not reject words without accents but will just tell you that you have a "typo" or "watch out for the accents".