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.
c _ baa _ aca _ cacab _ acac _ bca
The series is cab / aa / cacab / cacab / aa / cacab / ca.
Thus, the pattern cacab, cacab, aa is repeated.