**Core Concept**
The posterior belly of the digastric muscle is a part of the digastric muscle, which is a unique muscle with two bellies and a tendon that connects them. The digastric muscle plays a crucial role in the movement of the mandible, specifically in the depression and elevation of the mandible.

**Why the Correct Answer is Right**
The posterior belly of the digastric muscle is supplied by the facial nerve (cranial nerve VII). The facial nerve is responsible for controlling the muscles of facial expression, as well as providing sensory innervation to the face. The posterior belly of the digastric muscle is one of the few muscles that receive motor innervation from the facial nerve. This is because the digastric muscle is involved in the movement of the mandible, which is also controlled by the trigeminal nerve (cranial nerve V). However, the facial nerve supplies the posterior belly of the digastric muscle, making it a unique exception.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the anterior belly of the digastric muscle is supplied by the mylohyoid nerve, a branch of the inferior alveolar nerve, which is a branch of the mandibular division of the trigeminal nerve (cranial nerve V).
* **Option B:** Incorrect because the stylohyoid muscle is supplied by the facial nerve, but it is not the correct answer for the posterior belly of the digastric muscle.
* **Option D:** Incorrect because the digastric muscle's tendon is not supplied by the facial nerve.

**Clinical Pearl / High-Yield Fact**
The digastric muscle is a unique muscle that is involved in the movement of the mandible. It is one of the few muscles that receive motor innervation from the facial nerve, making it an important landmark for identifying the facial nerve during surgical procedures.

**Correct Answer: C. Facial nerve.**