Which of the following substances is classified as a stimulant?

Cocaine is classified as a stimulant due to its effect on the central nervous system, where it increases levels of dopamine, a neurotransmitter associated with pleasure and reward. This action leads to heightened alertness, increased energy, and enhanced mood, which are characteristic effects of stimulant substances.

In contrast, heroin is an opioid that primarily acts as a depressant, producing sedative effects rather than stimulating. Alcohol also functions as a depressant, impairing cognitive function, motor skills, and overall brain activity. Nicotine, while it has stimulating properties at low doses, is often better understood as a mild stimulant that can lead to a temporary increase in alertness and vigilance; however, cocaine is more classically recognized within the stimulant category due to its more potent and direct effects on the brain's neurotransmitters. Therefore, cocaine is the clear choice among the listed substances for classification as a stimulant.