The answer should be E.
The argument is based upon the fact that although the process of decaffeinating coffee beans is simple and not very costly,the retail price of decaffeinated coffee is considerably higher than that of regualr coffee.
The assumption the author makes is that beans used for decaffeinating does nt cost much compared to regular coffee.
If the beans used for decaffeinating was very high,then obviously it would have made sense that the price is high.
This is in fact the basis of the argument.

Here, the only reason that reach to the conclusion provided by author if the cost of the decaffeinated beans before processing is not more than the regular one's.

Only E says that. ....other options are far way from the point of reasoning.