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.
C _ B _ D _ A _ B B D D 2 _ _ 4 _ 3 4 _ ? ? ? ? _ a _ c b a _ d _ _ _ _
Clearly, 2 corresponds to C and 4 corresponds to A.
So, 1 and 3 corresponds to B and D.
Thus, the missing sequence is 1, 1, 3, 3 or 3, 3, 1, 1.