Classification – Letter pairs: choose the odd one out by comparing the forward shift from the first letter to the second (A=1 … Z=26). Three pairs advance by +4 positions; one pair advances by a different amount. Which pair is different? Options: GK, MQ, PU, SW.

Introduction / Context:
Letter-pair classifications often rely on a constant positional shift from the first to the second letter. Detecting the uniform step highlights the majority; any deviation becomes the odd one out.

Given Data / Assumptions:

• Alphabet index: A=1 … Z=26.
• Pairs: GK, MQ, PU, SW.
• Rule candidate: second letter position = first letter position + k (modulo 26), testing constant k.

Concept / Approach:
Compute the forward step for each pair and compare. Three pairs should share the same step; one will differ.

Step-by-Step Solution:
G(7) → K(11) = +4.M(13) → Q(17) = +4.S(19) → W(23) = +4.P(16) → U(21) = +5 (different).

Verification / Alternative check:
Reverse-check: subtract the second letter index from the first; only PU yields a +5 step, confirming the anomaly.

Why Other Options Are Wrong:
GK, MQ, SW: Each advances exactly +4 positions, so they fit the majority pattern.

Common Pitfalls:
Wrapping the alphabet unnecessarily (no wrap occurs here) or miscounting inclusive/exclusive steps.

Final Answer:
PU