OPTION A:- INCORRECT
This isn't the assumption which the argument is operating on because the main concern of the argument is not the dropped prices of the illegal drugs but how a program made to control the entry of illegal drugs into the country failed.

OPTION B:- INCORRECT
The argument's main concern isn't the prices of the drugs, neither is there any certainty that the price, neither is there any certainty that the price paid by a average consumer of the illegal drugs didn't drop substantially in 1987 as opposed by the argument itself.

OPTION C:- INCORRECT
The argument didn't claim or give any indication or prove to show that the domestic production of illegal drugs increased. Although the argument's concern is about drugs being brought into the country, not the ones being produced in it. So, this cannot serve as an assumption to which the argument is based on.

OPTION D:- INCORRECT
This contradicts the argument which claims that the wholesale price of most illegal drugs dropped substantially in 1987. So, the argument isn't based on this assumption.

OPTION E:- CORRECT
The drop in demand for the most illegal drugs in wholesale price. This is true because another reason for the drop in price is the failed attempt of the program made to control the entry of illegal drugs as the argument claims. Therefore, this is the assumption that the argument runs on.