Which of the following is a commonly known hallucinogen?

LSD, or lysergic acid diethylamide, is classified as a hallucinogen due to its profound effects on perception, mood, and cognitive processes. It primarily alters how the brain interprets sensory information, leading to visual and auditory hallucinations as well as an altered sense of time and self. It is specifically known for its ability to induce experiences often referred to as "trips," which can be either pleasurable or distressing.

In comparison, cocaine is a powerful stimulant that primarily affects mood and energy levels but does not typically induce hallucinations. Methamphetamine is also a stimulant, known for increasing alertness and energy, and can lead to severe psychological issues, but it doesn't fall within the category of hallucinogens. Barbiturates are central nervous system depressants that can induce sedation and sleep but again, do not produce hallucinations. Thus, LSD distinctly stands out in this context as a well-known hallucinogenic substance.