Which of the following substances is classified as a hallucinogen?

Mescaline is classified as a hallucinogen due to its ability to alter perception, mood, and various cognitive processes. Upon ingestion, mescaline acts on serotonin receptors in the brain, leading to visual and auditory hallucinations, changes in thought patterns, and an altered sense of reality. This classification places it distinctly in a category of substances that induce profound changes in the perception of the world, unlike the other substances listed.

Caffeine, while it is a psychoactive stimulant, primarily affects alertness and energy levels rather than causing hallucinations. Alcohol is a depressant that influences mood and coordination, and although it can alter perception in some ways, it does not produce the same transcendent, hallucinogenic effects as mescaline. Nicotine is a stimulant affecting the nervous system, known for its addictive properties and impact on alertness, but it does not induce hallucinations like mescaline does. The primary distinguishing feature of mescaline is its capability to invoke significant alterations in perception, aligning it with the traits characteristic of hallucinogens.