Find the missing number?

Answer: Option A

Taking numbers in diagonally opposite squares, calculate their sum, and put the answer in the small, inner square of the upper number. Calculate their difference, and put this answer in the small, inner square of the lower number. So,

3 + x = 12
x = 9.

Or.
x - 3 = 6
x = 9.
Missing Element = 9.