Which of the following substances is classified under hallucinogens?

LSD (lysergic acid diethylamide) is classified as a hallucinogen due to its psychoactive properties that significantly alter perception, mood, and cognitive processes. Hallucinogens, like LSD, affect the serotonin receptors in the brain, leading to altered thoughts, visual perceptions, and experiences that are often described as surreal or dream-like.

In contrast, the other substances listed serve different purposes: heroin is an opioid that primarily causes pain relief and euphoria; caffeine is a stimulant that enhances alertness and reduces fatigue; and Valium (diazepam) is a benzodiazepine used primarily for its sedative effects to treat anxiety and relax muscles. These classifications reflect their primary effects and mechanisms of action in the body, distinguishing them clearly from hallucinogens like LSD.