**Core Concept**
The question is testing the student's knowledge of the muscles involved in vocal cord movement. The correct answer requires an understanding of the anatomy and physiology of the larynx, specifically the muscles responsible for vocal cord abduction.

**Why the Correct Answer is Right**
The posterior cricoarytenoid muscle (PCA) is the only muscle that abducts the vocal cords. It is innervated by the recurrent laryngeal nerve, a branch of the vagus nerve. The PCA muscle originates from the posterior surface of the cricoid cartilage and inserts into the muscular process of the arytenoid cartilage. When it contracts, it rotates the arytenoid cartilage and pulls the vocal cord laterally, resulting in abduction.

**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is involved in vocal cord adduction and relaxation, not abduction.
* **Option B:** The cricothyroid muscle is responsible for tensing the vocal cords and increasing pitch, but not abduction.
* **Option C:** The arytenoid muscle is involved in adduction of the vocal cords and opening the glottis, but not abduction.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only muscle that abducts the vocal cords, and its paralysis can lead to vocal cord fixation and respiratory compromise.

**Correct Answer:** C. Posterior cricoarytenoid muscle