Find the missing number?

Answer: Option D

In each diagram, the sum of the top two and middle digits equals the sum of the lower 2 digits. I.e
Sum of Upper elements + middle element = Sum of the lower elements

In first Diagram,
5 + 3 + 12 = 14 + 6 = 20
Second Diagram,
4 + 4 +2 = 5 + 5 = 10
Third Diagram,
6 + 1 + 8 = 12 + x
x = 3.
Required Element = 3.