Which of the following letter-clusters will replace the question mark (?) in the given series? PK, GT, XC, OL, ?

GT HS EV FU

FU

Explanation: PK, GT, XC, OL, ? The logic followed here is : P → (-9) → G → (-9) → X → (-9) → O → (-9) → F K → (+9) → T → (+9) → C → (+9) → L → (+9) → U   So, the letter-cluster that replaces the (?) to complete the series is "FU"