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.
b _ abbc _ bbca _ bcabb _ ab
The series is bcab / bcab / bcab / bcab / bcab.
Thus, the pattern bcab is repeated.