Not necessary, because if processing regular coffee does NOT cost more than processing decaffeinated coffee, that can still support the author’s position that the extra price of decaf coffee isn’t explained by the cost of providing decaf to the consumer. For example, the processing cost of regular coffee might be the same as that for decaf — that supports the claim that the difference in price must be due to something besides the cost of providing decaf coffee.

Not necessary, because the author doesn’t propose any particular factors that would explain the price difference. So the author doesn’t have to believe that supply and demand are explanatory factors either generally or in the case of coffee. Maybe there are other reasons for the price difference besides supply and demand and besides production costs.

Not necessary, because “competition” has no role in the reasoning of the argument. Even if there’s a lot of competition among companies that process decaffeinated coffee, that doesn’t suggest anything about the reason for the price difference between regular and decaf.

Not necessary, because what retail sellers expect is irrelevant to the argument. We already know the retail price of decaf is higher than that of regular coffee. Whether sellers expect consumers to be content with that or not doesn’t change the fact that there is a price difference, nor does it suggest anything about the reason for that price difference.

Necessary, because if the beans used for producing decaf coffee DO cost much more before processing than the beans used for producing regular coffee, then the price difference MIGHT be accounted for by the greater cost of “providing” decaf coffee. (E) points out that there could be other components of “providing” coffee besides merely the cost of processing the coffee beans. The author must assume that these other components — such as the price of the bean itself — do not explain the price difference between decaf and regular.