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

ZBQS, WXLM, ? , QPBA, NLWU

TTGG

Explanation:

ZBQS, WXLM, ? , QPBA, NLWU

The logic followed here is :

Z → (-3) → W → (-3) → T → (-3) → Q → (-3) → N

B → (-4) → X → (-4) → T → (-4) → P → (-4) → L

Q → (-5) → L → (-5) → G → (-5) → B → (-5) → W

S → (-6) → M → (-6) → G → (-6) → A → (-6) → U

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