Find the missing number?

Answer: Option A

As you move from left to right, the sum of the digits in each circle forms a sequence of multiples of 6, from 6 to 30. So,

2 + 1 + 3 = 6
8 + 2 + 2 = 12
9 + 6 + 3 = 18
4 + 6 + 14 = 24.
So,

11 + 6 + x = 30
x = 13.