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 _ bc _ a _ bcda _ ccd _ bcd _
The series is aabcd / abbcd / abccd / abcdd.
Thus, a, b, c and d are repeated twice one by one.