Which one will replace the question mark?

Sum of numbers in lower half of the circle = 11 + 9 + 3 + 7 = 30

Sum of numbers in upper half of the circle = ? + 4 + 5 + 6 = ? + 15

Upper half = Lower half

30 = ? + 15

? = 30 - 15

? = 15.

Threfore, 15 is the answer.