Which of the following is an example of a hallucinogen?

LSD, also known as lysergic acid diethylamide, is classified as a hallucinogen because it significantly alters perceptions, mood, and cognitive processes. Hallucinogens are substances that can induce hallucinations, which are sensory experiences that appear real but are created by the mind. LSD specifically affects serotonin receptors in the brain, leading to changes in thought patterns, visual and auditory distortions, and profound shifts in reality perception.

In contrast, caffeine is a stimulant that primarily increases alertness and energy. Xanax, or alprazolam, is a benzodiazepine used to treat anxiety, which works by enhancing the effects of a neurotransmitter called GABA, leading to sedation rather than hallucination. Oxycodone is an opioid painkiller that works by binding to specific receptors in the brain to reduce the perception of pain, and it does not typically produce hallucinations. Thus, among these options, LSD stands out as the clear choice for an example of a hallucinogen.