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.
abb _ baa _ a _ bab _ aba
The series is abba / baab / abba / baab / a.
Thus, the pattern abba, baab is repeated.