**Core Concept**
The platysma is a superficial muscle of the neck that plays a key role in facial expressions. Its motor innervation is derived from the cervical plexus, which is a complex network of nerves that arise from the spinal cord. Understanding the specific nerve supply to the platysma is crucial for evaluating facial and neck anatomy.

**Why the Correct Answer is Right**
The platysma muscle receives its motor innervation from the cervical branch of the **vagus nerve** (CN X) via the **ansa cervicalis**, a nerve loop that connects the cervical plexus to the vagus nerve. This unique nerve supply allows for the coordination of facial expressions with other voluntary movements. The ansa cervicalis is formed by the fusion of the **C1-C3** spinal nerves, which contribute to the cervical plexus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the facial nerve (CN VII) primarily innervates the muscles of facial expression, excluding the platysma.
* **Option B:** This option is incorrect because the accessory nerve (CN XI) is primarily involved in the innervation of the sternocleidomastoid and trapezius muscles.

**Clinical Pearl / High-Yield Fact**
The platysma muscle is often used as a landmark for identifying the **anterior border of the sternocleidomastoid muscle**. This is a useful clinical correlation for students to remember, as it helps in visualizing the anatomy of the neck.

**Correct Answer: C. The cervical branch of the vagus nerve via the ansa cervicalis.**