Which of the following is classified as a hallucinogenic drug?

LSD, or lysergic acid diethylamide, is classified as a hallucinogenic drug because it primarily alters perception and can induce vivid hallucinations and changes in thought processes. Hallucinogens work by affecting the serotonin receptors in the brain, leading to altered sensory perceptions, enhanced emotional experiences, and sometimes profound psychological changes. Users often report experiencing changes in visual and auditory perceptions, along with an altered sense of time and self.

Cannabis, while it can have psychoactive effects, is primarily classified as a cannabinoid due to its active compounds, which do not typically induce hallucinations in the way that LSD does. Heroin, being an opioid, is mainly associated with pain relief and euphoria but does not primarily affect perception to the level of producing hallucinations. Alcohol is a depressant that alters coordination and judgment but does not induce hallucinations either. Thus, LSD is distinctly recognized in classifications of drugs as a hallucinogen, making it the correct answer.