"It is fine."
Translation:Mae hi'n braf.
A good place to check to see if words have been mutated or not is on gweiadur.com - you need to register but it is free - which is a very good on-line dictionary. On iOS or Android, Ap Geiriaduron will also point you to unmutated forms if you put in a mutated word.
This Duolingo course is a little naughty, in my opinion, in sometimes giving mutated words as a single-word question or answer.