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.
cccbb _ aa _ cc _ bbbaa _ c
The series is cc bbb aaa / ccc bbb aaa / c.
Thus, the pattern ccc bbb aaa is repeated.