Which letter replaces the question mark?

Concept: In letter-grid series puzzles, the letters lie along one continuous alphabetical sequence that advances by a fixed step, wrapping from Z back to A when it runs past the end. The same fixed step first builds one path through the figure (an outer boundary), and then the identical step is reused to build a second, smaller path (an inner square); the missing letter is found by continuing that established step from its immediate predecessor.

Application:

1. Trace the outer boundary of the whole 4x4 arrangement clockwise starting from the bottom-left corner: A(1) to F(6) to K(11) to P(16) to U(21) to Z(26) to E(5) to J(10) to O(15) to T(20) to Y(25) to D(4). Each move advances exactly 5 letters, wrapping from Z back to E.

2. This confirms the fixed step size to reuse for the inner 2x2 square. Trace the inner square's own clockwise ring starting at its bottom-left cell: K(11) to P(16) to U(21) to the missing letter. Continuing the same +5 step from U(21) gives 21 + 5 = 26 = Z.

3. Cross-check: the four inner values 11, 16, 21, 26 all leave the same remainder (1) when divided by 5, confirming they form one consistent progression, so Z is the letter that completes it.

Answer: Z