Which one will replace the question mark ?

Concept: In this circle-based number series, the four numbers placed around each circle (top, bottom, left, right) combine through one fixed rule to give the number at the centre. The rule is discovered from the two fully-worked circles and then applied to the incomplete one.

Applying the rule:

1. First circle: (Right − Left) + (Top − Bottom) = (15 − 12) + (10 − 9) = 3 + 1 = 4 — matches its centre value 4.

2. Second circle: (28 − 12) + (16 − 20) = 16 + (−4) = 12 — matches its centre value 12.

3. Third circle: (23 − 11) + (15 − 16) = 12 + (−1) = 11.

Cross-check: regrouping the same four numbers as (Top + Right) − (Left + Bottom) gives the identical result for every circle — (10 + 15) − (12 + 9) = 4, (16 + 28) − (12 + 20) = 12, and (15 + 23) − (11 + 16) = 11 — confirming the value independently.

So the number that replaces the question mark is 11.