It is a very interesting piece because it deals with only lexical construction.

Option A - Incorrect
The supply couldn't have dropped because the program to control the incessant supply of illegal drugs could not be achieved. It could be argued that since be argued that since the stoppage couldn't be achieved then it's wrong to assert that it dropped "significantly".

Option B - Incorrect
It is going to drop substantially because it's production has been embraced by locals too and not just dependent on legal entry.

Option C - Incorrect
This is the only reason why the price would not have dropped. i.e . Local production of illegal drugs has been increased significantly than what now comes into the country.

Option D - Incorrect
The assertion could be right because the wholesale price of "most" illegal drugs decreased not "all" which in fact strengthens the author's suggestion that few could have increased substantially.

Option - correct
Very true because domestic production peaked. This is actually responsible directly for a drop in demand of illegal drugs.