라면 is correct and 라멘 is false.
In English however the word ramen is a loanword from Japanese.
Ramyeon is the romanization for the Korean word 라면, but not the translation.
So we pronounce and write 라면(ramyeon) in Korean, but we translate it to ramen in English.
In my personal opinion though, I think of the Japanese ramen (like tonkotsu) when I see the word ramen. So I don't really think it's the best translation.
I usually just say (Korean) instant noodles, since 라면 in Korean is none other than instant noodles (and sometimes the noodles only without the soup powder, for example in army stew). I haven't tried it in duolingo though, because I wish to remain my streak :p