An example of a substitute good would be a
sale on skis in the summer.
sale on swimsuits in the winter.
lower-priced gallon of milk.
higher-priced luxury car.

A substitute good is a product that makes you want to buy it more than another product when you actually need it. So, if there are two stores selling the same goods, but one is cheaper, usually you are going to go there to buy that particular product. Having that in mind, the correct answer is lower-priced gallon of milk. A and B are incorrect because you don't really need skis in the summer or swimsuits in the winter, and if a luxury car is more expensive than it used to be, the demand is going to fall.