Solution
Based on the English alphabet, the above arrangement follows the following pattern :
The first alphabet in each word is
C --(+6)--> I --(+6)--> O --(+6)--> U
The second alphabet is
C --(+2)--> E
$\therefore$ U --(+2) --> W
And the third alphabet is
C --(-2)--> A
$\therefore$ U --(-2) --> S
Thus, UWS wil be the next letter in the series.