**Core Concept**
The spinal accessory nerve (cranial nerve XI) plays a crucial role in shoulder movement, particularly in rotating the head against resistance, also known as the "rotator test." This test assesses the integrity of the spinal accessory nerve, which innervates the sternocleidomastoid and trapezius muscles.

**Why the Correct Answer is Right**
The spinal accessory nerve is responsible for innervating the sternocleidomastoid and trapezius muscles, which are essential for rotating the head against resistance. When the patient is unable to perform this action, it suggests an injury to the spinal accessory nerve. The sternocleidomastoid muscle helps in rotating the head to the opposite side, while the trapezius muscle assists in elevating the scapula and rotating the scapula downward. The spinal accessory nerve provides motor innervation to these muscles, facilitating this movement.

**Why Each Wrong Option is Incorrect**
**Option A:** The posterior auricular nerve (cranial nerve X) primarily innervates the muscles of the auricle and is not involved in rotating the head against resistance.
**Option B:** The 10th cranial nerve (vagus nerve) is responsible for various functions, including parasympathetic innervation of the thoracic and abdominal organs, but not for rotating the head against resistance.
**Option D:** The 12th cranial nerve (hypoglossal nerve) primarily innervates the muscles of the tongue and is not involved in rotating the head against resistance.

**Clinical Pearl / High-Yield Fact**
The spinal accessory nerve can be injured during surgical procedures like neck dissections, or it can be damaged due to trauma or radiation therapy to the neck. Knowledge of this nerve's function in rotating the head against resistance is essential for diagnosing and managing patients with spinal accessory nerve palsy.

**✓ Correct Answer: C. Spinal accessory nerve**