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.

GOOD : HSXT :: BEST : CIBJ :: COOL : ?

DSXB

Pattern followed is :

GOOD : HSXT

G(7)+1 = 8(H) , O(15) + 4 = 19(S) , O(15) + 9 = 24(X) , D(4) + 16 = 20(T)

BEST : CIBJ

B(2) + 1 = 3(C) , E(5)+4 = 9 (I), S(19) +9 = B , T(20) + 16 = J

Similarly 

COOL : ?

C(3) + 1 = 4(D) , O(15) + 4 = (19)S , O(15) + 9 = 24(X) , L(12) + 16 = B

DSXB , is correct answer .