**Core Concept**
Buprenorphine is a medication used for managing opioid use disorder and pain relief. It acts on the opioid receptors in the brain and spinal cord to produce analgesia and prevent withdrawal symptoms.

**Why the Correct Answer is Right**
Buprenorphine is a partial agonist at the mu-opioid receptor (MOR). This means it binds to the receptor and activates it, but to a lesser extent than full agonists like morphine. As a result, buprenorphine produces analgesia and reduces withdrawal symptoms, but it also has a ceiling effect, which limits its potential for overdose and abuse. The partial agonist activity of buprenorphine is thought to be due to its unique binding mode, which allows it to occupy the receptor without fully activating it.

**Why Each Wrong Option is Incorrect**
**Option A:** Buprenorphine is not a full agonist at the delta-opioid receptor (DOR), which is involved in pain modulation and emotional processing.
**Option B:** Buprenorphine is not a pure antagonist at the kappa-opioid receptor (KOR), which is involved in pain modulation and mood regulation.
**Option C:** Buprenorphine is not a full agonist at the sigma-opioid receptor (SOR), which is involved in pain modulation and modulation of other opioid receptors.

**Clinical Pearl / High-Yield Fact**
Buprenorphine's partial agonist activity makes it a safer alternative to full opioid agonists for managing chronic pain and opioid use disorder, but it can still cause dependence and withdrawal symptoms if used improperly.

**Correct Answer:** C.