I believed there is a typo error in the choices. Below I assume that the correct choices:
A. 9 (3+4)
B. 9 (4+5)
C. 7(4+5)
D. 7(3+6)
The answer is letter A which is 9 (3+4) the greatest common denominator of 27 and 36 is 9. The factor of 27 is 9 and 3 while for 36 is 9 and 4. To check the answer you need to solve 9 (3+4) which is 63 while 27 + 36 is also 63.