Which of the following drugs is an example of a hallucinogen?

LSD, or lysergic acid diethylamide, is a well-known example of a hallucinogen. Hallucinogens are substances that primarily affect the user's perception, mood, and various cognitive processes, often leading to altered states of consciousness or visual and auditory hallucinations. LSD specifically is known for its profound effects on sensory experiences and thought processes, which is characteristic of this class of drugs.

In contrast, other substances listed, such as alcohol, cocaine, and heroin, belong to different drug categories. Alcohol is classified as a depressant, primarily affecting the central nervous system to slow down brain activity. Cocaine is a stimulant, which increases alertness and energy by accelerating the brain's function. Heroin is classified as an opioid and is primarily associated with pain relief and euphoria but does not induce hallucinations. Thus, among the choices given, LSD distinctly fits the definition of a hallucinogen.