Either works, and they're both accepted.
"Which" is usually preferred if a choice is being made between a number of options, while "what" is used when it's an open question. Norwegian makes no such distinction, and we have no context here to indicate that one translation should be better than the other.
Duolingo is oriented towards American English rather than Britihsh English. For example: there is American flag in the English course, preference for "fall" over "autumn", and "colors" over "colours". I wondered why is it that way, since British is the 'original' Engilsh that should be taught. But eventually I have accepted it as it is ;) I guess it's because the founder of Duolingo is American.