Find the missing number?

Answer: Option D

1st element in 3rd circle = 1st element in 1st circle + 1st element in second circle = 1 + 6 =7
2nd element in 3rd circle = 2nd element in 1st circle + 2nd element in second circle = 3 +5 = 8
Same way,
2 +6 = 8 and
7 + 1 = 8.

In 4th Circle,
1st element = Difference of 1st element of 1st circle and 2nd circle = 6 -1 = 5
2nd element = Difference of 2nd element of 1st circle and 2nd circle = 5 -3 = 2
3rd element = Difference of 3rd element of 1st circle and 2nd circle = 7 - 1 = 6.
So, required element = 6.