Find the missing number?

Answer: Option B

In each star, calculate the difference between the 2 left hand and 2 right hand numbers.
The value at the top of each star equals the product of these differences, and the value at the bottom of each star equals the difference between these 2 differences.

So,
7 - 6 = 1 and
4 - 2 = 2.
Now,
2 * 1 = 2 (Top element in lower star).
And,
2 - 1 = 1 ( Lower element in Lower star).
Missing Element = 1.