Which letter-cluster will replace the question mark (?) to complete the given series?

KRPD, ?, QNRL, TLSP, WJTT

NPQH

Explanation:

KRPD, ?, QNRL, TLSP, WJTT

The logic followed here is :

K → (+3) → N → (+3) → Q → (+3) → T → (+3) → W

R → (-2) → P → (-2) → N → (-2) → L → (-2) → J

P → (+1) → Q → (+1) → R → (+1) → S → (+1) → T

D → (+4) → H → (+4) → L → (+4) → P → (+4) → T

So, the letter-cluster that replaces the (?) to complete the series is "NPQH"