**Core Concept**
The question is testing the student's knowledge of the muscles involved in the movement of the vocal cords, specifically the abductors. The abductors are responsible for moving the vocal cords apart, which is essential for breathing and speech production.

**Why the Correct Answer is Right**
The correct answer is the posterior cricoarytenoid muscle. This muscle is the only abductor of the vocal cords and is responsible for rotating the arytenoid cartilages laterally, thereby moving the vocal cords apart. This movement is crucial for inspiration and is essential for maintaining a patent airway. The posterior cricoarytenoid muscle is innervated by the vagus nerve and is the only muscle that abducts the vocal cords, making it a vital component of the laryngeal musculature.

**Why Each Wrong Option is Incorrect**
**Option A:** The arytenoid muscle is involved in adduction of the vocal cords, not abduction. It is innervated by the recurrent laryngeal nerve and plays a key role in vocal cord closure.

**Option B:** The lateral cricoarytenoid muscle is involved in adduction of the vocal cords, not abduction. It is also innervated by the recurrent laryngeal nerve and works in conjunction with the arytenoid muscle to close the vocal cords.

**Option D:** The thyroarytenoid muscle is involved in relaxation of the vocal cords, not abduction. It is also innervated by the recurrent laryngeal nerve and plays a role in vocal cord closure, but not in abduction.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is often referred to as the "singing voice muscle" because it is responsible for the precise movement of the vocal cords necessary for singing. Damage to this muscle can result in voice disorders and breathing difficulties.

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