"What dessert is your favorite" should be correct.
"Which dessert is your favorite?" should also be accepted
what is the dessert you like the most?
should be correct too right?
can i say 甜点 as 'Sweets'?
What is your most favourite dessert? Wasn't accepted!
It says "最". In other cases in this course, if you don't translate "最" it was wrong. Why then is it wrong now? This translation should be perfectly acceptable.
What dessert do you prefer
"What is your favorite kind of dessert?" should be correct.
Duolingo is usually pretty lenient with spelling errors, but I guess 'what is your favourite desert' is a question in its own right. I think it might be the Sahara...