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 _ cdaab _ cc _ daa _ bbb _ ccddd
The series is abca / aabbccdd / aaabbbcccddd.
Thus, each letter of first sequence is repeated two times in the second sequence and three times in the third sequence.