**Core Concept**
The Carotid Triangle, also known as the Carotid Triangle of the neck, is an anatomical region bounded by several structures that are crucial for the understanding of neck anatomy. This triangle is clinically significant due to the presence of vital structures such as the common carotid artery, internal jugular vein, and the vagus nerve.

**Why the Correct Answer is Right**
The posterior boundary of the Carotid Triangle is formed by the Sternocleidomastoid muscle. This muscle plays a crucial role in the movement of the neck and head. It originates from the sternum and clavicle and inserts into the mastoid process of the temporal bone. The Sternocleidomastoid muscle divides the neck into anterior and posterior triangles.

**Why Each Wrong Option is Incorrect**
**Option A:** The common carotid artery is not the posterior boundary of the Carotid Triangle; it is actually one of the structures that passes through the triangle.
**Option B:** The internal jugular vein is another structure that passes through the Carotid Triangle but does not form its posterior boundary.
**Option C:** The vagus nerve is a vital structure that passes through the Carotid Triangle but does not form its posterior boundary.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the Sternocleidomastoid muscle plays a significant role in the movement of the neck and head, and its division of the neck into anterior and posterior triangles is crucial for clinical examination and surgical procedures.

**Correct Answer:** C. Sternocleidomastoid muscle