Select the option that is related to the sixth letter cluster in the same way as the first letter cluster is related to the second letter cluster and third letter cluster is related to fourth letter cluster.
OLB : JGW :: CWM : XRH :: ? : CIK

Given: OLB : JGW :: CWM : XRH :: ? : CIK

Logic: Letters are decreasing - 5 place.

For, OLB : JGW

O - 5 = J, L - 5 = G, B - 5 = W

For, CWM : XRH 

C - 5 = X, W - 5 = R, M - 5 = H

Similarly,

? : CIK

C + 5 = H, I + 5 = N, K + 5 = P

So, HNP : CIK

Thus, correct option is (b).