d.) Ron in washing, neither in mowing.
Work Step by Step
A "comparative advantage" is the ability to produce a good at a lower opportunity cost than another producer. Since Ron can wash more cars in an hour, he can wash a car in less time than David. Therefore, his opportunity cost--specifically the cost of his time--is lower than David's. Since they mow the same number of lawns in an hour, neither has a comparative advantage in mowing.