**Core Concept**
The question is testing the student's knowledge of the muscles involved in vocal cord movement. The abductor muscles are responsible for separating the vocal cords, which is essential for breathing. The primary abductor of the vocal cord is a muscle that plays a crucial role in regulating airflow during respiration.

**Why the Correct Answer is Right**
The correct answer is the posterior cricoarytenoid muscle, which is the only muscle that abducts the vocal cords. This muscle originates from the lateral aspect of the cricoid cartilage and inserts into the muscular process of the arytenoid cartilage. It acts by rotating the arytenoid cartilage, which in turn separates the vocal cords. This action is essential for inhalation, as it allows the vocal cords to move apart and increase the diameter of the glottis.

**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is an adductor and relaxant of the vocal cord, not an abductor.
* **Option B:** The cricothyroid muscle is a tensor of the vocal cord, which increases the tension and pitch of the vocal cord, but it does not abduct the vocal cord.
* **Option D:** The arytenoid muscle is not a primary abductor of the vocal cord; it is involved in the adduction and rotation of the arytenoid cartilage.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only muscle that abducts the vocal cord, and its paralysis can lead to vocal cord fixation and respiratory compromise. This muscle should be evaluated in patients with suspected vocal cord paralysis or respiratory distress.

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