In an alphabet-mirror code, each letter is replaced by its counterpart from the opposite end of the English alphabet. Choose the cluster that completes the analogy:

? : LT :: AH : ZS :: NR : MI

The correct option is OG.

Step 1: Understanding the Alphabet-Mirror Code
In an alphabet-mirror code (also known as opposite-letter coding), each letter is replaced by its counterpart from the reverse order of the English alphabet (A ↔ Z, B ↔ Y, C ↔ X, etc.). A useful rule for mirror letters is that the sum of their numerical positions in the alphabet is always 27:

$\text{Position of Original Letter}+\text{Position of Mirror Letter}=27$

Step 2: Analyzing the Given Analogy Pairs
Let's verify the rule using the provided pairs:

1. For NR : MI
- N is the 14^th letter, and its mirror M is the 13^th letter:
$14+13=27$
- R is the 18^th letter, and its mirror I is the 9^th letter:
$18+9=27$

2. For AH : ZS
- A is the 1^st letter, and its mirror Z is the 26^th letter:
$1+26=27$
- H is the 8^th letter, and its mirror S is the 19^th letter:
$8+19=27$

Step 3: Finding the Missing Cluster (?) for LT
We need to determine which cluster mirrors to LT:

- First letter: The mirror counterpart of L (12^th letter) is found by:
$27-12=15$
The 15^th letter of the English alphabet is O.

- Second letter: The mirror counterpart of T (20^th letter) is found by:
$27-20=7$
The 7^th letter of the English alphabet is G.

Conclusion:
Combining these two letters gives OG. Thus, OG : LT completes the analogy pattern.