Two multiple choice answers for the translation of “ongelofelijk” are “unbelievable” and “incredible”. Both are correct answers. But according to google translate, “unbelievable” is the correct translation. But the app marks it incorrect and only allow “incredible” as the translation.
hi there, how courageous you are to learn NL ! 
first thing: ongelovelijk / ongelofelijk / ongelooflijk are just the same.
Second : http://www.interglot.com/dictionary/nl/en/translate/ongelooflijk
this is another confirmation of your search… Dus alles is goed, neen 
You know a word in a language does have a single translation in a another.
Both translation are quite similar.
Now as far as the app is concerned well we should say the course… you know is a kind of robot. So if the synonyms are not defined in the course it will not find out. Now you understand It would be quite a big job to define all synonyms in a course…
If it’s really annoying you I would recommend to address you request to the course creator so that he can help you or correct that in the course.
Nog proficiat !
en tot…
Thanks for your response Chris. I am told by Memrise to write this up as a bug so that it would be picked up by the course developer to fix it 
You are right, both are perfectly correct answers. The App is wrong/incomplete.