Which letter correctly replaces the question mark ?

Answer: X

Explanation: Read the letters following a snake (boustrophedon) path across the grid: go left-to-right on the first row, then move down one row and go right-to-left, then down and left-to-right, and so on.

• Step-size pattern: advance through the alphabet by 2, then 3, then 4, up to 8, and then repeat this sequence (2,3,4,5,6,7,8,2,3...).

• Apply the step sizes in order while following the snake path starting at the top-left letter. For example: D +2 → F, F +3 → I, I +4 → M, and so on; continuing this process along the full path places X at the missing cell.

Therefore the correct letter in the question mark position is X.