Which of the following is classified as a stimulant?

Caffeine is classified as a stimulant because it acts on the central nervous system to increase alertness, elevate mood, and enhance energy levels. As an adenosine receptor antagonist, caffeine blocks the effects of adenosine, a neurotransmitter that promotes sleep and relaxation. This results in increased neuronal activity and the release of other neurotransmitters such as dopamine and norepinephrine, contributing to its stimulating effects.

In contrast, the other substances listed operate in different ways. Diazepam is a benzodiazepine that acts as a sedative and anxiolytic, primarily depressing the central nervous system. Alcohol is also a depressant, which slows down brain functions and can lead to decreased inhibitions and impaired judgment. GHB (gamma-hydroxybutyrate) has sedative properties and can induce sleep, further identifying it as a depressant rather than a stimulant. Thus, caffeine stands out as the only substance among the options that enhances central nervous system activity and promotes wakefulness.