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 _ aa _ bbb _ aaa _ bbba
The series is abb / aaabbb / aaaabbbb / a.
Thus, the letters are repeated twice, then thrice, then four times and so on.