Which of the following is a major class of psychoactive substances?

The correct choice encompasses all listed options, as each represents a major class of psychoactive substances.

Cannabinoids, which include compounds found in marijuana, exert effects on the brain and affect mood, memory, and appetite. Stimulants, such as cocaine and amphetamines, enhance alertness and energy levels by increasing the activities of certain neurotransmitters in the brain. Opioids, including substances like heroin and prescription pain relievers such as morphine and oxycodone, interact with opioid receptors to produce pain relief, euphoria, and can lead to dependence and addiction.

By recognizing that each of these classes has distinct characteristics and effects on the central nervous system, it becomes evident that they all are classified as psychoactive substances. Therefore, the collective response of "All of the above" is correct, as it acknowledges the significance and impact of all these substances on psychoactive properties.