Which of the following is a common example of a narcotic?

Heroin is classified as a narcotic and is an opioid derived from morphine. It acts on the body's opioid receptors, leading to pain relief and feelings of euphoria, which makes it highly addictive. The term "narcotic" generally refers to substances that have sedative and analgesic properties, which are characteristic of heroin. It is often used in discussions regarding controlled substances due to its high potential for abuse and the significant health risks associated with its use.

In contrast, cocaine is a stimulant rather than a narcotic; it increases energy and alertness rather than providing sedation. Alcohol is also a depressant but is not categorized as a narcotic; it affects the central nervous system differently. LSD, on the other hand, is a hallucinogen and alters perception and mood without the sedative effects typical of narcotics. This understanding emphasizes why heroin is the correct answer, as it fits the definition and characteristics associated with narcotics more closely than the other options presented.