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

BRH, ZUD, ? , VAV, TDR, RGN

XXZ

Explanation:

BRH, ZUD, ? , VAV, TDR, RGN

The logic followed here is :

B → (-2) → Z → (-2) → X → (-2) → V → (-2) → T  → (-2) → R

R → (+3) → U → (+3) → X → (+3) → A → (+3) → D  → (+3) → G

H → (-4) → D → (-4) → Z → (-4) → V → (-4) → R  → (-4) → N

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