The correct option is D 18
Factor pairs of 36:
1x36 = 36;
2x18 = 36;
3x12 = 36;
4x9 = 36;
6x6 = 36
Factors of 36 are 1,2,3,4,6,9,12,18,36
Factor pairs of 54:
1x54 = 54;
2x27 = 54;
3x18 = 54;
6x9 = 54
Factors of 54 are 1,2,3,6,9,18,27,54
Common factors are 1,2,3,6,9,18
Highest common factor = 18