Which of the following drugs falls under hallucinogens?

LSD, or lysergic acid diethylamide, is classified as a hallucinogen due to its profound effects on perception, mood, and cognitive processes. Hallucinogens are substances that alter a person's sensory perceptions, leading to experiences such as visual and auditory hallucinations. LSD works primarily by interacting with serotonin receptors in the brain, which is responsible for mood and perception regulation.

The other substances mentioned do not fit the category of hallucinogens. Nicotine is a stimulant found in tobacco that primarily affects the central nervous system and can enhance alertness. Ethanol, commonly known as alcohol, is classified as a depressant that slows down brain function and motor skills. Caffeine, a widely consumed stimulant found in coffee and tea, primarily increases alertness and energy levels without inducing hallucinations.