What should come in place of question mark (?) in the given series based on…

Concept: In a letter-series puzzle built on the English alphabet, every three-letter block is produced from the block before it by shifting all three of its letters by the identical number of alphabet positions, cycling past either end of the alphabet (after Z the count continues from A, and before A it continues from Z). Numbering each letter by its alphabet position turns the letter pattern into a simple arithmetic step that must repeat identically across all three positions at once.

Application: Number each letter (A = 1, B = 2, … Z = 26) and track the three letter-columns across the four given terms:

1. 1st letters: 7 → 3 → 25 → 21. Each step is −4 places, cycling past A back to Z (3 − 4 = −1, which cycles to 25).

2. 2nd letters: 9 → 5 → 1 → 23. The same −4 step repeats (1 − 4 = −3, cycling to 23).

3. 3rd letters: 14 → 10 → 6 → 2. The same −4 step repeats once more.

4. Since all three columns follow one identical, cycling −4 step, that shared step is the rule governing the whole series — apply it once more to the last term's three letters to find the missing block.

Cross-check: Applying −4 to U (21) gives 17 = Q; applying −4 to W (23) gives 19 = S; applying −4 to B (2) gives −2, which cycles to 24 = X. Each result was reached by the same −4 step already confirmed three times over across GIN→CEJ, CEJ→YAF, and YAF→UWB, so the check does not depend on any single column.

Result: The missing term is QSX.