"I am wearing a coat and a hat."
Translation:Dw i'n gwisgo cot a het.
That's also the spelling used when the word is introduced in https://www.duolingo.com/skill/cy/Clothes - both in the Tips and Notes and, as I recall, in the section itself.
The GPC dictionary lists it as "côt, cot", so apparently both spellings should be acceptable.
All the examples there of noun compounds with that word use the "côt" spelling, though.
It would be good if the course standardised on one spelling for this word and accepted both.