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.
ba _ b _ aabb _ a _ _ a _ bb
The series is baab / baab / baab / baab / b.
Thus, the pattern baab is repeaated.