Which letter replaces the question mark?

Concept: In this style of letter-grid puzzle, treat each letter as its position in the English alphabet (A = 1, B = 2, ..., Z = 26). Such a grid follows one fixed arithmetic relationship across every row — the value of one entry equals the sum of the other two entries in that row.

1. Convert every letter to its alphabet position: Row 1 is A(1), G(7), F(6); Row 2 is J(10), Y(25), O(15); Row 3 is E(5), H(8), and the missing letter.

2. Test Row 1: the first and third entries add to 1 + 6 = 7, which is exactly the second entry, G. So the rule is first + third = second.

3. Confirm with Row 2: the first and third entries add to 10 + 15 = 25, which is exactly the second entry, Y — the same rule holds.

4. Apply the confirmed rule to Row 3: first + third = second gives 5 + missing value = 8, so the missing value = 8 - 5 = 3, the 3rd letter of the alphabet, C.

Cross-check: E (5) + C (3) = 8 = H, so Row 3 follows exactly the same first + third = second pattern already verified in Rows 1 and 2.

Hence, the missing letter is C.