What is a common hallucinogen?

LSD, or lysergic acid diethylamide, is a well-known hallucinogen that is primarily used for its psychoactive effects, which can include altered perceptions of reality, visual hallucinations, and changes in mood and thought processes. As a classic hallucinogen, LSD interacts with serotonin receptors in the brain, particularly the 5-HT2A receptor, leading to its characteristic effects on perception and cognition.

The other substances listed do not share these hallucinogenic properties. Cocaine is a powerful stimulant that primarily increases dopamine levels, resulting in feelings of euphoria and increased energy but not hallucinations. Hydroxybutyric acid, commonly known as GHB, is classified as a depressant and is used for its sedative and euphoric effects, rather than inducing hallucinations. Oxycodone is an opioid pain reliever that produces analgesic effects and can lead to sedation and euphoria but does not typically cause hallucinations.

Thus, LSD stands out as a classic example of a hallucinogen, making it the correct answer in this context.