Which letter replaces the question mark?

In an alphabet-grid puzzle like this, each row is built by moving forward through the alphabet using one fixed step (wrapping from Z back to A when needed), and the step size itself increases by exactly one letter as you move from one row to the next.

1. Find the step used within each of the first five rows by comparing consecutive letters: Row 1 advances by 4 letters each time (H, L, P, T, X, B, F, J), Row 2 by 5 (A, F, K, P, U, Z, E, J), Row 3 by 6, Row 4 by 7, and Row 5 by 8.

2. Since the step grows by exactly 1 from row to row (4, 5, 6, 7, 8, ...), Row 6 must use a step of 9.

3. Row 6 reads D, M, V, E, N, W, F, ? -- the letter just before the question mark is F, the 6th letter of the alphabet.

4. Moving forward 9 letters from F (position 6) lands on position 15, which is the letter O.

This step of 9 holds across the whole row, not just the final jump -- D to M, M to V, V to E (wrapping past Z), E to N, N to W, and W to F are each exactly 9 letters apart, so applying the same step once more from F confidently gives O.