Find the missing number?

Answer: Option C

Clock-wise:
First Square:
2 + 3 + 6 + 9 = 20.

Second Square:
6 + 7 + 3 + 3 = 19

Fourth Square:
6 + 9 + 1 + 1 = 17

Third Square:
1 + 3 + x + 5 = 18
x = 18 - 9 = 9.
Missing Number = 9.