Which letter replaces the question mark ?

Concept

In a snake-path (boustrophedon) alphabet grid, only the shaded cells carry letters; tracing them row by row — left-to-right, then down one row and right-to-left, alternating — gives one continuous sequence. In this family of puzzles, each letter advances over the previous one by a step size that itself increases from a small value up to a maximum, and once that maximum is reached the step size resets back to its smallest value and the increasing pattern repeats.

Application

Reading only the filled cells of the grid in the snake order (row 1 left-to-right, row 2 right-to-left, row 3 left-to-right, and so on) gives the path: D, F, I, M, R, X, E, M, O, R, V, A, G, N, V, and finally the missing entry. Tracking the alphabet-position gained at each step:

1. D to F: +2

2. F to I: +3

3. I to M: +4

4. M to R: +5

5. R to X: +6

6. X to E: +7

7. E to M: +8 (one full 2-to-8 cycle is now complete)

8. M to O: +2 (the cycle restarts)

9. O to R: +3

10. R to V: +4

11. V to A: +5

12. A to G: +6

13. G to N: +7

14. N to V: +8 (a second full cycle is now complete)

15. V to the missing letter: +2 (a third cycle begins) — V is the 22nd letter, so 22 + 2 = 24th letter

The 24th letter of the alphabet is X.

Cross-check

The reset-to-+2 transition already appears once earlier in the path: right after E to M completed the first full 2-through-8 cycle, the very next step (M to O) resets to +2. The missing entry occupies the same position in the pattern — right after N to V completed a second full cycle — so the same +2 reset applies, giving V + 2 = X, independently confirming the keyed answer.

Answer: X.