Which letter replaces the question mark?

Concept: In a letter-grid series, all the letters around the border are read as a single continuous alphabetical sequence — start at a fixed corner and travel one way around the perimeter. The gap between consecutive letters keeps growing by one letter at every step (skip 1, then 2, then 3, and so on), wrapping from Z back to A when needed.

Application: Starting at J (top-left) and moving anti-clockwise (down the left side first), the perimeter reads: J, L, O, S, X, D, K, S, B, L, W, I, V, J, ?, O. Using A = 1 … Z = 26 and tracking the growing gap:

1. J (10) to L (12): gap +2 (skip K).

2. L (12) to O (15): gap +3 (skip M, N).

3. O (15) to S (19): gap +4 (skip P, Q, R).

4. S (19) to X (24): gap +5 (skip T, U, V, W).

5. X (24) to D (4): gap +6, wrapping past Z (skip Y, Z, A, B, C).

6. D (4) to K (11): gap +7 (skip E, F, G, H, I, J).

7. K (11) to S (19): gap +8.

8. S (19) to B (2): gap +9, wrapping past Z.

9. B (2) to L (12): gap +10.

10. L (12) to W (23): gap +11.

11. W (23) to I (9): gap +12, wrapping past Z.

12. I (9) to V (22): gap +13.

13. V (22) to J (10): gap +14, wrapping past Z.

14. J (10) to the missing letter: gap +15, so 10 + 15 = 25 = Y.

Cross-check: The pattern must keep holding for the letter right after the missing one too. From Y, the next gap is +16: 25 + 16 = 41, which wraps to 41 − 26 = 15 = O — and O is exactly the next letter in the grid (the left-hand letter of the top-middle cell). Both the incoming and outgoing gaps confirm the missing letter is Y.