I need help with this problem

The third one. This is because Proportional relationships are relationships between two or more (3 in your case) variables where their ratios are equivalent. Another way to think about them is that, in a proportional relationship, one variable is always a constant value times the other. That constant is know as the "constant of proportionality".

You can see that the slope is constant as 1/3 among all your points.

The answer is d 18ft