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 C _ B D _ C D C D 2 _ 4 1 _ 1 4 _ _ _ _ c d _ b c _ a ? ? ? ?
Clearly, 4 corresponds to C and a corresponds to 4.
So, a corresponds to C.
1 corresponds to D and b corresponds to 1.
So, b corresponds to D.
Thus, CDCD corresponds to a, b, a, b.