Which of the following letter-clusters will replace the question mark (?) and complete the following letter-cluster series?

IR, BY, UF, NM, GT, ?

ZA

Explanation:

IR, BY, UF, NM, GT, ?

The logic followed here is :

I → (-7) → B → (-7) → U → (-7) → N → (-7) → G → (-7) → Z

R → (+7) → Y → (+7) → F → (+7) → M → (+7) → T → (+7) → A

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