Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
SA, OU, KO, ? ​​

SA, OU, KO, ?

Logic: Each term, 1st letter decreased by 4 and 2nd letter decreased by 6.

S - 4 = O

O - 4 = K

A - 6 = U

U - 6 = O

So, GI is next term.

Thus, the correct answer is (a).