Which one will replace the question mark ?

In circle-based number puzzles, the four numbers placed around the circle (top, bottom, left and right) are combined by a fixed arithmetic rule to produce the center number; the rule is found by testing it on the circles that are already solved, then applied to the circle with the missing number.

1. First circle: check (Right - Left) + (Top - Bottom) = (15 - 12) + (10 - 9) = 3 + 1 = 4, which matches the given center value 4.

2. Second circle: apply the same rule = (28 - 12) + (16 - 20) = 16 - 4 = 12, which matches the given center value 12, so the rule is confirmed on both solved circles.

3. Third circle: apply the confirmed rule to its own numbers = (23 - 11) + (15 - 16) = 12 - 1 = 11.

Cross-check by elimination: applying the same confirmed rule to the third circle's own numbers gives exactly one value; testing the rule's arithmetic against each of the other three offered values for this circle shows none of them satisfy it, so the value obtained is the unique one consistent with the pattern already validated on both solved circles.

Therefore, the missing number is 11.