"The boy has a cake."
Translation:Cậu bé có một cái bánh.
The boy has a cake. I said ....mot banh and the correct answer was mot cai banh???
Một cái bánh or một chiéc bánh is correct. Một bánh is ungrammatical. In the case of bread loaves, you can say chiếc bánh or ổ bánh. By the way the word bánh here is a generic term for all kinds of sweet, tart or savoury cakes, pastries, biscuits and other snacks.