Expression : RP, DM, SQ, CL, TR, ? The pattern given in each letter of the alternate terms is : Odd terms : 1st letter : R (+1 letter) = S (+1 letter) = T 2nd letter : P (+1 letter) = Q (+1 letter) = R Even Terms : 1st letter : D (-1 letter) = C (-1 letter) = B 2nd letter : M (-1 letter) = L (-1 letter) = K Thus, missing term = BK