72^2y = 5184y
18xy = 18xy
72^2y factors (only some of them not All of them): 1, 2, 3, 4, 6, 8, 9, 12, 16, 18, y
18xy factors: 1, 2, 3, 6, 9, 18, x, y
Your answer: 18xy (Option D.)
Note: Just rewrite the factors of the problem until you find out what you're looking for in the particular problem
Good Luck!