Soldiers returning to the united states who used high-quality while in vietnam discontinued use when they were no longer confronted by the anxiety and depression of the war experience.
a. cocaineb. marijuanac. alcohold. heroin

d. heroin

Explanation:

In the early 1970's, there was argued to be a distinctive heroin problem among  American troops in Vietnam.  

It was discovered that there were heroin addicts from over 15 percent of  American troops deployed there.  

The finding resulted in a burst of activity, including the establishment of  President Nixon's Special Action Office for Drug Abuse Prevention,  promoting prevention and recovery, and tracking addicted military members back  home.

this doesn't support enough information to answer the question.

i think it is the third one.