I pick E.
(A) Changes in the level of drug use by high school seniors are seldom matched by changes in the level of drug use by other people below the age of 20.
<The statement says that the survey sometimes matches the level of drug use by other people below 20.>
(B) In the past, high school seniors were consistently the population group most likely to use illegal drugs and most likely to use them heavily.
<out of scope, talks about the past and how seniors were consistently most likely to use drugs and shows no reason why the number of seniors using drugs is declining>
(C) The percentage of high school seniors who use illegal drugs is consistently very similar to the percentage of all people below the age of 20 who use illegal drugs.
<shows no reason why the number of seniors using drugs is declining >
(D) The decline revealed by the surveys is the result of drug education programs specifically targeted at those below the age of 20.
<out of scope, the passage talks nothing about drug education, In addition this statement talks bout people under 20 NOT high school seniors.>
(E) The number of those surveyed who admit to having sold illegal drugs has declined even faster than has the number who have used drugs.
Correct <Those who were surveyed and admitted to selling drugs said they were selling less drugs than the number who use them. So if the supply of drugs is declining faster the seniors who use them, the number of seniors who use drugs will decline cause they cannot obtain the drugs.>