Select the letter-pair from among the given options that can replace the question mark (?) in the following series.

FU, GT, HS, ?

Given: FU, GT, HS, ?

Logic: First Letter + 1 and  Second Letter - 1 

F + 1 = G

G + 1 = H

U - 1 = T

T - 1 = S

Now, the next term will be IR.

Thus, the correct option is (a) IR.