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.
aba _ baca _ ba _ bacaabac _ aca
The series is abac / baca / abac / baca / abac / baca.
Thus, the pattern abac, baca is repeated.