Which of the following substances is classified as a hallucinogen?

LSD, or lysergic acid diethylamide, is classified as a hallucinogen because it has the ability to alter perception, mood, and a variety of cognitive processes. Hallucinogens work by affecting the serotonin receptors in the brain, which leads to changes in sensory experiences and can result in hallucinations, where users may see, hear, or feel things that are not present.

LSD is well-known for its profound mental effects, which can include visual distortions and an altered sense of time and self. This distinguishes it from substances like caffeine, nicotine, and alcohol, which primarily act as stimulants or depressants rather than hallucinogens. Caffeine and nicotine can stimulate the central nervous system but do not produce hallucinations or significant perceptual changes. Alcohol, although it affects mood and cognitive function, typically acts as a depressant rather than a hallucinogen.