The correct option is C 9
Factors of 27 are 1,3,9 and 27.
Factors of 36 are 1,2,3,4,6,9,12,18 & 36.
The common factors of 27 and 36 are 3 and 9.
The largest number among these is 9.
This is called the Greatest Common Factor or Greatest Common Divisor of 27 and 36.