Which of the following is a narcotic antagonist?

Naloxone is classified as a narcotic antagonist because it has the ability to block the effects of opioids at their receptor sites in the brain. It specifically binds to the same receptors as narcotic agonists, such as morphine and fentanyl, but does not activate them. This action makes naloxone particularly valuable in clinical settings for reversing opioid overdoses or mitigating the effects of opioid medications.

In contrast, the other substances listed serve different roles in the context of narcotic pharmacology. Fentanyl is a potent opioid agonist, widely used for its analgesic properties. Buprenorphine is a partial agonist, meaning it activates opioid receptors but to a much lesser extent, making it useful for pain management and treatment of opioid dependence while carrying a lower risk of misuse. Morphine is a full agonist and is commonly used for pain relief.

Thus, naloxone's unique blocking action on opioid receptors qualifies it as a narcotic antagonist, distinguishing it from the other options that either activate these receptors or interact with them in a different manner.