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.
adb _ ac _ da _ cddcb _ dbc _ cbda
The series is adbc acbd abcd dcba dbca cbda.
Thus, the letters equidistant from the beginning and the end of series are the same.