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

RSBG, NPEI, ?, FJKM, BGNO

JMHK

Explanation:

RSBG, NPEI, ?, FJKM, BGNO

The logic followed here is :

R → (-4) → N → (-4) → J → (-4) → F → (-4) → B

S → (-3) → P → (-3) → M → (-3) → J → (-3) → G

B → (+3) → E → (+3) → H → (+3) → K → (+3) → N

G → (+2) → I → (+2) → K → (+2) → M→ (+2) → O

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