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.
a _ abbb _ ccccd _ ddccc _ bb _ ba
The series is aaa / bbbb / cccc / dddd / cccc / bbbb / a.