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.
_ cb _ ca _ bacb _ ca _ bac _ d
The series is acbd / cadb / acbd / cadb / acbd.
Thus, the pattern acbd / cadb is repeated.