The argument's conclusion is that the program to control the entry of illegal drugs into the country was a failure in 1987. This conclusion is based on the evidence that the wholesale price of most illegal drugs dropped substantially in 1987. For this argument to hold, it assumes that the drop in wholesale prices was due to an increase in supply rather than a decrease in demand.
This assumption is directly addressed in: (E) A drop in demand for most illegal drugs in 1987 was not the sole cause of the drop in their wholesale price.
This choice states that the price drop was not only because of reduced demand, implying that supply (including illegal entry) was a factor, supporting the argument's conclusion that the program failed to control drug entry.