**Core Concept**
The posterior triangle of the neck is an anatomical region bounded by the sternocleidomastoid and trapezius muscles. The fascia overlying this region serves as a protective barrier for the structures within, including nerves, blood vessels, and lymphatic tissues. Understanding the relationships between these structures and the overlying fascia is crucial for identifying potential sites of injury or disease.

**Why the Correct Answer is Right**
The correct answer involves the spinal accessory nerve (cranial nerve XI), which traverses the posterior triangle of the neck. This nerve emerges from the jugular foramen, passes through the posterior triangle, and enters the sternocleidomastoid muscle, where it receives motor innervation. The fascia overlying the posterior triangle is pierced by the spinal accessory nerve as it traverses this region. This is due to the nerve's superficial course through the triangle, which makes it susceptible to injury.

**Why Each Wrong Option is Incorrect**
**Option A:** The external jugular vein is a superficial structure that passes through the parotid gland and does not traverse the posterior triangle of the neck.
**Option B:** The subclavian artery and vein are located more medially and do not pierce the fascia over the posterior triangle.
**Option C:** The accessory nerve does not pierce the fascia over the anterior triangle of the neck.

**Clinical Pearl / High-Yield Fact**
The spinal accessory nerve is a critical structure for motor innervation of the sternocleidomastoid and trapezius muscles, which play essential roles in shoulder movement and posture.

**Correct Answer:** C.