**Core Concept**
The gag reflex, also known as the pharyngeal reflex, is a **physiological reflex** that helps prevent foreign objects from entering the airway. It involves the **cranial nerves** and the **brainstem**. The reflex has an afferent limb, which senses the stimulus, and an efferent limb, which responds to the stimulus.

**Why the Correct Answer is Right**
The efferent limb of the gag reflex is mediated by the **vagus nerve (CN X)**, which is responsible for the motor response. When the **pharynx** or **larynx** is stimulated, the afferent limb sends signals to the **brainstem**, which then activates the efferent limb, causing **contraction of the pharyngeal muscles** and **elevation of the larynx**.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it is not a cranial nerve involved in the motor response of the gag reflex.
**Option B:** is incorrect because it is not the primary nerve responsible for the efferent limb.
**Option D:** is incorrect because it is not directly involved in the gag reflex.

**Clinical Pearl / High-Yield Fact**
The gag reflex is an important **protective mechanism** that helps prevent aspiration.

**Correct Answer:** C. Vagus nerve (CN X)