**Core Concept**
The cranial accessory nerve (CN XI) is a branch of the accessory nerve, which provides motor innervation to several muscles in the head and neck. The cranial root of the accessory nerve joins the vagus nerve and contributes to the formation of the pharyngeal plexus, which innervates several muscles involved in swallowing.

**Why the Correct Answer is Right**
The muscles of the pharynx, including the stylopharyngeus, are innervated by the pharyngeal plexus, which is formed by the cranial root of the accessory nerve (CN XI) and the vagus nerve (CN X). This innervation is responsible for the coordinated movement of the pharynx during swallowing.

**Why Each Wrong Option is Incorrect**
**Option A:** The sternocleidomastoid and trapezius muscles are the primary muscles of the neck that are innervated by the spinal accessory nerve, making this an incorrect choice.
**Option B:** The infrahyoid muscles, including the sternohyoid, omohyoid, and sternothyroid, are innervated by the ansa cervicalis, which is a branch of the cervical plexus, not the accessory nerve.
**Option C:** The laryngeal muscles, including the cricothyroid, are innervated by the external laryngeal nerve, a branch of the vagus nerve (CN X), not the accessory nerve.

**Clinical Pearl / High-Yield Fact**
When assessing patients with dysphagia or difficulty swallowing, it is essential to evaluate the function of the pharyngeal muscles, which are innervated by the pharyngeal plexus. The stylopharyngeus muscle, in particular, plays a crucial role in the initiation of swallowing.

**Correct Answer:** D. Infrahyoid muscles