**Core Concept**
The question tests the understanding of opioid-induced muscle rigidity, a well-known side effect of potent opioid analgesics like fentanyl. This phenomenon is related to the action of opioids on specific receptors in the central nervous system. **Mu (μ) receptors** are the primary target for opioid-induced analgesia and side effects.

**Why the Correct Answer is Right**
Fentanyl, a potent **μ receptor** agonist, induces muscle rigidity by stimulating these receptors in the central nervous system. The **μ receptors** are G-protein coupled receptors that, when activated, can lead to decreased release of inhibitory neurotransmitters, thereby increasing the tone of muscles and resulting in rigidity. This effect is particularly pronounced with high doses of potent opioids like fentanyl.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while other receptors may play roles in various opioid effects, they are not primarily responsible for muscle rigidity induced by fentanyl.
**Option B:** Incorrect as it does not accurately represent the receptor type associated with fentanyl-induced muscle rigidity.
**Option C:** Incorrect because, although opioids have actions on multiple receptor types, muscle rigidity is most closely associated with **μ receptor** activation.
**Option D:** Incorrect as it is not the correct receptor type responsible for this specific side effect of fentanyl.

**Clinical Pearl / High-Yield Fact**
Remember, **μ receptor** agonism is key to both the therapeutic and side effects of opioids like fentanyl. Muscle rigidity is a significant consideration in clinical settings, particularly in anesthesia, where high doses of opioids are used.

**Correct Answer:** D. Mu receptor.