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 _ _ bb _ a _ bbab _ ba
The series is babb / babb / babb / babb / babb / ba.
Thus, the pattern babb is repeated.