Find the missing number?

Answer: Option D

Calculate the sums of the numbers in the left and right hand circles, to give 2 digit answers.
Write the left hand answer in the top 2 segments of the central circle, and put the right hand answer in the bottom 2 segments of the central circle. So,
3 + 6 + 1 + 8 + 9 + 2 = 29.
4 + 4 + 3 + 2 + 7 + 1 = 21.
So,

Missing element = 21.