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.
a _ bbc _ aab _ cca _ bbcc
The series is aabbcc / aabbcc / aabbcc.
Thus, the pattern aabbcc is repeated.