What is the next letter? y b w d u f

The correct answer is S. The sequence is each alternate letter, changing from one end of the alphabet to the other.

Z Y X W V U T S R Q P L M N O K J I H G F E D C B A
Since we started at the end of the alphabet and then went back to the beginning, there are two letters in the middle that aren't part of the pattern. We only need to worry about which letter is next at the end of the alphabet, though, and the next highlighted letter after U is S.

Hope this helps!