Answer:
What you said as D is correct.
Mike thought it was the other way around, so how much of y is x, instead of how much of x is y. C would be correct if x and y were flipped around.
Explanation:
What you thought was the correct answer does happen to be correct. Divide y by x to get the constant.
12 / 2 = 6
18 / 3 = 6
24 / 4 = 6!
The constant is 6 every time, so D is correct.
Mike here chose C as his answer, which is completely wrong. You divide y by x to get the constant, not x by y. However, there is an alternate way C would be correct. If the three coordinates switched their x and y values ((2, 12) is now (12, 2), (3, 18) is now (18, 3), (4, 24) is now (24, 4)), then C is correct.