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.
ab _ bcbca _ _ c _ bab
The series is abcbc / bcaca / cabab.
Thus, the series consists of three sequences.
The first sequence begins with a, the second with b and the third with c.
Each sequence consists of a letter followed by other two letters repeated twice.