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.
ac _ cab _ baca _ aba _ acac
The series is acac / abab / acac / abab / acac.
Thus, the pattern acac, abab is repeated.