Which of the following is an example of a stimulant?

Cocaine is classified as a stimulant due to its ability to increase the levels of dopamine in the brain, which leads to heightened feelings of energy, alertness, and euphoria. Stimulants generally enhance activity in the central nervous system, resulting in increased heart rate, blood pressure, and improved focus.

In contrast, the other substances listed have different classifications. Alcohol is a depressant, which slows down brain function and depresses the central nervous system. PCP, while sometimes thought of as a hallucinogen, can also have depressant effects. Barbiturates, like alcohol, are also classified as depressants and are used mainly to sedate or calm individuals. Thus, cocaine stands out among the options as the only stimulant.