Which letter replaces the question mark?

In each circle of this series, the three letters — read clockwise starting from the top-left segment (top-left, then top-right, then bottom) — form an arithmetic progression of alphabet positions: the alphabet-position gap from the first letter to the second equals the gap from the second letter to the third, within that same circle (treating positions cyclically, wrapping from Z back to A when a sum exceeds 26).

1. First circle (A, F, K): the gap from A (1) to F (6) is 5, and the same gap of 5 applied from F gives 6 + 5 = 11 = K, confirming the equal-gap rule for this circle.

2. Second circle (D, J, P): the gap from D (4) to J (10) is 6, and the same gap applied from J gives 10 + 6 = 16 = P.

3. Third circle (B, I, P): the gap from B (2) to I (9) is 7, and the same gap applied from I gives 9 + 7 = 16 = P.

4. Fourth circle (O, W, E): the gap from O (15) to W (23) is 8, and the same gap applied from W gives 23 + 8 = 31, which wraps past Z (26) to 31 − 26 = 5 = E.

5. Fifth circle (F, O, ?): using the same within-circle rule, the gap from F (6) to O (15) is 9, so applying that same 9-position gap from O gives 15 + 9 = 24, the position of X.

This also matches the pattern across circles: the gap rises by exactly one from circle to circle through the whole series (5, 6, 7, 8, 9), independently confirming the same 9-position gap for this circle.

The missing letter is X.