The third and fourth rules combine to give us two basic diagrams:
T _ _ _ _ _ _ 1 2 3 4 5 6 7
R _ _ _ _ _ T 1 2 3 4 5 6 7
This question tells us that the fifth sale is on headphones. In order to separate S from H according to rule 1, there is only one option for S, which is week 2. This allows us to place P in week 3.
T S P _ H _ _ 1 2 3 4 5 6 7
R S P _ H _ T 1 2 3 4 5 6 7
We also know that L is before H.
T S P L H _ _ 1 2 3 4 5 6 7
R S P L H _ T 1 2 3 4 5 6 7
This gives us only a few options for M. In diagram 1, it could be in 6 or 7, before or after R.
T S P L H R/M R/M 1 2 3 4 5 6 7
In diagram 2 it must be in spot 6, before T.
R S P L H M T 1 2 3 4 5 6 7
T is not one of the answer choices here, so the sale of microwaves can only be earlier than the sale on refrigerators, as seen in diagram 1. This is why D is correct.
