**Core Concept**
The phenomenon of ear pain during tonsillitis is a classic example of referred pain, which occurs due to the shared nerve supply between the tonsils and the ear. The glossopharyngeal nerve (cranial nerve IX) provides sensory innervation to both the tonsils and the pharynx, including the area behind the ear.

**Why the Correct Answer is Right**
The correct answer involves the glossopharyngeal nerve, which transmits pain signals from the tonsils to the brain. This nerve has a common pathway with the auriculotemporal nerve, which innervates the area around the ear. As a result, pain from the tonsils can be referred to the ear, causing ear pain during tonsillitis. This phenomenon is known as convergence of pain pathways.

**Why Each Wrong Option is Incorrect**
* **Option A:** The vagus nerve (cranial nerve X) is involved in the parasympathetic innervation of the pharynx, but it is not responsible for the pain sensation in the ear during tonsillitis.
* **Option B:** The facial nerve (cranial nerve VII) is responsible for facial expressions and taste, but it does not innervate the tonsils or the ear.
* **Option C:** The hypoglossal nerve (cranial nerve XII) is involved in the motor innervation of the tongue, but it is not related to the pain sensation in the ear during tonsillitis.

**Clinical Pearl / High-Yield Fact**
The phenomenon of referred pain during tonsillitis is a classic example of how pain can be perceived in a different location from the actual source of the pain. This is an important concept to remember in clinical practice, as it can help healthcare providers to accurately diagnose and manage patients with referred pain.

**Correct Answer:** C. Glossopharyngeal nerve.