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.
ab _ aa _ caab _ c _ abb _ c
The series is abc / aabc / aabbc / aabbcc.
First all the letters occur once, then a occurs twice, then both a and b occur twice and finally all the three letters appear twice.