What should come in place of ? in the given series based on the English…

Concept: Every letter of the English alphabet has a fixed position number, A = 1 up to Z = 26. In a letter series each letter-slot of a term carries its own arithmetic progression of position numbers, and the counting is cyclic: once a position number crosses 26 you subtract 26 to come back to A. So the series is read column by column in position numbers, never as words.

Application: Convert every given term into its three position numbers.

Now read each column downwards.

1. First-letter column: 2, 8, 14, 20 — a constant step of +6. The next value is 20 + 6 = 26, and position 26 is Z.

2. Second-letter column: 5, 11, 17, 23 — again a constant step of +6. The next value is 23 + 6 = 29, which crosses 26, so 29 − 26 = 3, and position 3 is C.

3. Third-letter column: 23, 3, 9, 15 — the same step of +6, with one wrap already inside the given terms (23 + 6 = 29, 29 − 26 = 3). The next value is 15 + 6 = 21, and position 21 is U.

Cross-check: All three columns advance by the same step of +6, so the whole term moves uniformly. Stepping backwards by 6 from 26, 3 and 21 gives 20, 23 and 15 — exactly T, W and O, the term already printed before the blank. The missing term is therefore ZCU.

Watch out: The wrap is where this series is usually lost. The third column already wraps inside the given terms — W = 23 moves to C = 3, not to a letter further down the alphabet — so the constant step of +6 only becomes visible after the letters are converted to position numbers. And 29 is not a letter: subtract a full 26 to bring it back to position 3, and read that as C; reducing by 25 or by 27 instead lands on a neighbouring letter and derails the whole term.