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

TRO, RQM, PPK, NOI, ?

LNG

Explanation:

Explanation:

TRO, RQM, PPK, NOI, ?

The logic followed here is :

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

R → (-1) → Q → (-1) → P → (-1) → O → (-1) → N 

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

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