Which letter replaces the question mark ?

In this figure, letters are arranged in two rings — an outer ring of eight letters and an inner ring of eight letters — with one inner letter on the same spoke as each outer letter. Assign every letter its alphabet position (A = 1, B = 2, ..., Z = 26). Each outer letter is linked to the inner letter that lies diametrically opposite it (four spokes away, on the far side of the ring), and every such opposite pair adds up to the same fixed total.

1. Confirm the fixed total using pairs that are already complete: M (13) is opposite G (7), giving 13 + 7 = 20; Q (17) is opposite C (3), giving 17 + 3 = 20. So the constant for this ring is 20.

2. Locate the inner letter diametrically opposite the missing outer letter (the question mark): it is F (6).

3. Solve for the missing letter: 20 - 6 = 14, and the 14th letter of the alphabet is N.

The same constant holds for every other opposite pair in the figure — A (1) opposite S (19) gives 1 + 19 = 20, and O (15) opposite E (5) gives 15 + 5 = 20 — confirming that the missing letter is N.