Letter series type of questions usually consists of a series of small letters which follow a certain pattern. However, some letters are missing from the series. These missing letters are then given in a proper sequence as one of the alternatives. The candidate is requitred to choose this alternative as the answer.
abca _ bcaab _ aa _ caa _ c
The series is a / bcaa / bcaa / bcaa / bcaa / bc.
Thus, the pattern bcaa is repeated.