**Core Concept**
The question is testing the knowledge of the laryngeal muscles and their functions, specifically the action of abduction of the vocal cords. The abductor muscles are responsible for moving the vocal cords apart, which is essential for breathing.

**Why the Correct Answer is Right**
The posterior cricoarytenoid muscle is the only muscle in the larynx that acts as an abductor of the vocal cords. This muscle originates from the posterior aspect of the cricoid cartilage and inserts into the arytenoid cartilage. When it contracts, it rotates the arytenoid cartilage, which in turn abducts the vocal cords. This action is crucial for opening the glottis and allowing air to enter the lungs during inspiration.

**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is a tensor of the vocal cords, not an abductor. It helps to shorten and tense the vocal cords, which is essential for speech and singing.
* **Option B:** The lateral cricoarytenoid muscle is a tensor and adductor of the vocal cords, not an abductor. It helps to rotate the arytenoid cartilage and bring the vocal cords together.
* **Option D:** The arytenoid muscle is not a primary muscle of the larynx and does not have a significant role in the abduction of the vocal cords.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only muscle in the larynx that acts as an abductor of the vocal cords. This muscle is also the only muscle that is innervated by the recurrent laryngeal nerve, which is a branch of the vagus nerve.

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