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.
_ c _ bd _ cdcda _ a _ db _ a
The series is acdb / dacb / cdab / acdb / da.
The third letter in each sequence becomes the first letter in the following sequence.