Which of the following letter-number clusters will replace the question mark…

Concept: In an alpha-numeric series every term carries two independent channels. The letters are decoded by their alphabet positions (A = 1, B = 2, … Z = 26) and each letter slot advances by a fixed step that cycles back to A after Z, while the trailing number advances by a fixed difference. Fix the step for each letter slot and the difference for the number separately, then apply both rules to the last given term.

Application: The given series is DUS18, IZX34, NEC50, SJH66, ?, and decoding each term into alphabet positions gives the table below.

1. First letters: 4 → 9 → 14 → 19 rises by a constant step of +5, so the next first letter is 19 + 5 = 24, which is X.

2. Second letters: 21 → 26 → 5 → 10 also rises by +5, because 26 + 5 = 31 runs past Z and cycles to 31 − 26 = 5; the next second letter is therefore 10 + 5 = 15, which is O.

3. Third letters: 19 → 24 → 3 → 8 rises by +5 as well, because 24 + 5 = 29 cycles to 29 − 26 = 3; the next third letter is therefore 8 + 5 = 13, which is M.

4. Numbers: 34 − 18 = 16, 50 − 34 = 16 and 66 − 50 = 16, a constant difference of 16, so the next number is 66 + 16 = 82.

5. Putting the four channels back together gives XOM82.

Cross-check: Run the rules backwards from XOM82: 24 − 5 = 19 (S), 15 − 5 = 10 (J), 13 − 5 = 8 (H) and 82 − 16 = 66, which reproduces SJH66 exactly, so the constant letter step of +5 and the constant number difference of 16 hold across every pair of consecutive terms.