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.
_ bcdbc _ dcabd _ bcdbc _ dc _ bd
The series is abcd / bcad / cabd / abcd / bcad / cabd.
Thus, the pattern abcd / bcad / cabd is repeated twice.