"May I walk? Yes."
Translation:Ga i gerdded? Cei.
There are many reasons for a soft mutation. The ones that come up on this course are each explained in the 'Hints and Tips' for the section where each one comes up for the first time - see https://www.duolingo.com/comment/17638579
In this particular case, it is because cerdded is the object of the short-form verb ga ...?.
Which Yes/No responses are used depend on how the question is asked. The course notes are there to help - https://forum.duolingo.com/comment/17638579. For example - https://www.duolingo.com/skill/cy/Gai/tips-and-notes
There is a summary of the response system here - https://forum.duolingo.com/topic/924/hot and look for the sticky discussion on 'Saying yes and no'.