Select the option that is related to the fifth letter-cluster in the same way as the second letter-cluster is related to the first letter-cluster and the fourth letter-cluster is related to the third letter-cluster.

EXA : HSH :: LWQ : ORX :: KNO : ?

Logic: first letter is increasing by 3, second letter is decreasing by 5 and third letter is increasing by 7.

As,

EXA : HSH

E+3=H, X-5=S, A+7=H

And

LWQ : ORX

L+3=O, W-5=R, Q+7=X

Similarly,

KNO : ?

K+3=N, N-5=I, O+7=V

So, NIV is the missing letter-cluster.

Thus, correct option is (a).