Which of the following is an example of a narcotic?

Heroin is classified as a narcotic because it is an opioid substance that is derived from morphine and has potent analgesic (pain-relieving) properties. Narcotics, in general, are drugs that can depress the central nervous system and are typically used to manage severe pain. They also have the potential for abuse and can lead to physical dependence and addiction. Heroin specifically alters the brain's chemistry, producing feelings of euphoria and pain relief but also carries a high risk of overdose and a range of adverse effects on physical and mental health.

In contrast, tranquilizers are primarily used for anxiety and stress relief, alcohol is a legal depressant with psychoactive effects but is not classified as a narcotic, and barbiturates, while also central nervous system depressants, are not considered narcotics in the way that opioids are. Understanding the specific classifications and effects of these substances is crucial for law enforcement personnel in the context of drug enforcement and public safety.