In the following series of letter pairs, one pair is missing. Identify the correct pair that continues the pattern: ZA, TG, NM, ?

Introduction / Context:
This alphabet series question involves pairs of letters that move through the alphabet in a structured way. Your task is to discover the rule for how the first letters change and how the second letters change, then use the rule to find the missing pair. This type of problem strengthens pattern recognition skills used across many reasoning topics.

Given Data / Assumptions:

• Given pairs: ZA, TG, NM, ? • We work with the English alphabet, A = 1, B = 2, ..., Z = 26. • Movements can be forward or backward and can wrap around from Z to A. • The same arithmetic rule should apply step by step.

Concept / Approach:
We treat the first letters as one sequence and the second letters as another. We then convert each letter to its numerical position and look at the differences between consecutive terms. Once a consistent pattern is found for both positions, we extend it to determine the missing pair. This method avoids guesswork based only on visual similarity of letters.

Step-by-Step Solution:
Step 1: Translate first letters to numbers. First letters: Z (26), T (20), N (14). Differences: 26 → 20 is −6, 20 → 14 is −6. So we consistently subtract 6. Apply again: 14 − 6 = 8, which corresponds to H. Step 2: Translate second letters to numbers. Second letters: A (1), G (7), M (13). Differences: 1 → 7 is +6, 7 → 13 is +6. So we consistently add 6. Apply again: 13 + 6 = 19, which corresponds to S. Step 3: Combine the results. The missing pair is HS.

Verification / Alternative check:
The pattern is perfectly symmetric: the first letters move backward in the alphabet in steps of 6, while the second letters move forward in steps of 6. Using this rule from ZA to TG to NM, and then to HS, yields a smooth and consistent sequence. No other option fits both the first and second letter patterns simultaneously, which confirms that HS is the only valid answer.

Why Other Options Are Wrong:
• SH: This reverses the correct pair. It does not match the backward first letter and forward second letter pattern. • GT: Both letters break the exact −6 and +6 rules when checked against the previous term NM. • TG: This repeats an earlier term and does not show the necessary continuation of the backward and forward movements.

Common Pitfalls:
One common mistake is to look only at visual symmetry and select the reversed option SH without checking numeric positions. Another pitfall is to focus on one letter of the pair and ignore the second. Alphabet series questions usually require you to verify the pattern for both letters, otherwise apparently close answers can trick you.

Final Answer:
The letter pair that correctly completes the series is HS.