**Core Concept**
Opioid receptors are a class of G-protein-coupled receptors that play a crucial role in the analgesic, euphoric, and other effects of opioids. The opioid receptor system includes three main subtypes: mu (μ), delta (δ), and kappa (κ) receptors, each with distinct agonist and antagonist profiles.

**Why the Correct Answer is Right**
Naloxone is a pure antagonist of opioid receptors, specifically targeting the mu (μ) receptor subtype. It competes with opioid agonists, such as morphine and heroin, for binding sites on the μ receptor, thereby reversing their effects. Naloxone's mechanism of action involves rapid dissociation of opioid agonists from their receptors, which is thought to be responsible for its rapid onset of action and high efficacy in reversing opioid overdose.

**Why Each Wrong Option is Incorrect**
**Option A:** Naltrexone is a mixed opioid receptor antagonist with some partial agonist activity at the κ receptor subtype, making it less effective as a pure antagonist.
**Option B:** Pentazocine is a partial agonist at the κ receptor subtype and an antagonist at the μ receptor subtype, leading to complex pharmacological effects.
**Option C:** Buprenorphine is a partial agonist at the μ receptor subtype and an antagonist at the κ receptor subtype, making it less suitable as a pure antagonist.

**Clinical Pearl / High-Yield Fact**
Naloxone's effectiveness in reversing opioid overdose is highly dependent on its rapid administration, as it has a short half-life and may not provide sustained relief.

**Correct Answer:** C. Naloxone