The hints on hover all say this is a negative: They won't, they don't. I've reported an error here. Was I wrong?
This was wrong and was deleted. To see the latest version of the course you need to either delete your browser history, or to avoid affecting other sites you access, we suggest you install a browser just for Duolingo, such as Vivaldi, and delete the history periodically.
Fyddan nhw'n is the question form and would mean 'will they?' or 'will they be?'
With the addition of 'ddim' it becomes the negative.
Although it is a deleted (or disabled) sentence, it still seems to be appearing so I've put it back in the course with the answer 'will they' as the correct one. I'm not sure how long that will take to go live.