**Core Concept**
The patient is presenting with signs of opioid overdose, characterized by respiratory depression, miosis, and altered level of consciousness. Opioid overdose is a medical emergency requiring prompt treatment to prevent respiratory failure and death.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with opioid-induced respiratory depression, which is caused by the activation of mu-opioid receptors in the brainstem, leading to decreased respiratory drive. The administration of naloxone, a specific opioid antagonist, rapidly reverses opioid-induced respiratory depression by competitively inhibiting the action of opioids at mu-opioid receptors. This is the first-line treatment for opioid overdose.

**Why Each Wrong Option is Incorrect**
**Option A:** Nalbuphine is a mixed agonist-antagonist opioid, which is not effective in reversing opioid-induced respiratory depression and may even worsen the situation.
**Option B:** Flumazenil is a benzodiazepine antagonist, which is not indicated for opioid overdose and may actually precipitate seizures in patients with a history of benzodiazepine use.
**Option C:** Physostigmine is a cholinesterase inhibitor, which is not indicated for opioid overdose and may actually worsen respiratory depression by increasing parasympathetic tone.

**Clinical Pearl / High-Yield Fact**
In patients with opioid overdose, always administer naloxone in a monitored setting and be prepared to repeat doses as the effects of naloxone may wear off.

**Correct Answer:** C. Naloxone