Final answer:
The psychoactive drug from the list provided that is not a depressant is (D) Heroin, which is an opioid and not classified as a depressant like the others.
Step-by-step explanation:
The question asks which of the listed psychoactive drugs is not a depressant. A depressant is a type of psychoactive drug that tends to suppress central nervous system (CNS) activity.
Alcohol, barbiturates, benzodiazepines, and Nembutal are all examples of depressants, as they are agonists of the gamma-Aminobutyric acid (GABA) neurotransmitter system, which has a calming effect on the brain.
However, heroin is not classified as a depressant; it is classified as an opioid, which is a different class of psychoactive drugs. Heroin does produce sedating effects, but opioids primarily act by binding to opioid receptors in the brain, which can lead to pain relief and feelings of euphoria.
Therefore, the correct answer to the question is (D) Heroin.