Alphabet-pair series: AZ, GI, MN, __, YB — identify the missing pair by discovering the rule that governs the first and second letters across the series.

Introduction / Context:
In alphabet-pair series, each term consists of two letters. The rule can act on (i) the first letters across terms, (ii) the second letters across terms, or (iii) both, sometimes using symmetric or arithmetic patterns on letter positions (A=1, B=2, …, Z=26). The given series is: AZ, GI, MN, __, YB. We must determine the missing fourth pair.

Given Data / Assumptions:

• Pairs: A Z; G I; M N; ? ?; Y B.
• Alphabet positions: A=1, …, Z=26.
• We expect a consistent progression separately for first and second letters.

Concept / Approach:
Analyze the first letters: A(1), G(7), M(13), ?, Y(25). The jumps are +6, +6, +6, +6. So the fourth first-letter must be 13+6 = 19, which is S. Now analyze the second letters: Z(26), I(9), N(14), ?, B(2). Notice that in the 1st, 3rd, and 5th terms, the letters are end-symmetric pairs that sum to 27 with their partners (A+Z = 1+26 = 27; M+N = 13+14 = 27; Y+B = 25+2 = 27). This symmetry strongly suggests the missing 4th pair should also sum to 27: since the first letter is S(19), the partner must be 27-19 = 8, which is H.

Step-by-Step Solution:
1) First letters: A→G→M→?→Y are +6 each, so ? = S.2) Symmetry check on pairs that sum to 27: (A,Z), (M,N), (Y,B) → maintain pattern for the 4th pair.3) Compute second letter: 27 − S(19) = 8 → H.4) The missing pair is SH.

Verification / Alternative check:
Double-check that SH fits the observed structures: First-letter arithmetic (+6 ladder) holds; 4th pair also fits the recurring 27-sum symmetry observed among non-adjacent terms, keeping the sequence coherent.

Why Other Options Are Wrong:
RX, KF, TS break either the +6 rule for the first letters or fail the 27-sum symmetry constraint for the pair. Only SH satisfies both.

Common Pitfalls:
Focusing solely on one position or assuming both letters move by the same fixed step. Here, one track is a uniform +6, while the other is governed by a symmetric-sum rule.

Final Answer:
SH