**Core Concept**
The question is testing the anatomy and physiology of the larynx, specifically the muscles responsible for vocal cord movement. The abductor muscles are crucial for opening the glottis and facilitating breathing.

**Why the Correct Answer is Right**
The posterior cricoarytenoid muscle (PCA) is the only abductor of the vocal cord. It does so by rotating the arytenoid cartilages laterally, thereby abducting the vocal cords and opening the glottis. This action is essential for inspiration and maintaining a patent airway.

**Why Each Wrong Option is Incorrect**
**Option A:** The thyroarytenoid muscle is an adductor of the vocal cord, not an abductor. It helps in the closure of the glottis.
**Option B:** The cricothyroid muscle is a tensor of the vocal cord, which helps in changing the pitch of the voice, but it is not an abductor.
**Option C:** The lateral cricoarytenoid muscle is an adductor of the vocal cord, not an abductor. It helps in the closure of the glottis.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is often referred to as the "singing voice" muscle because it is responsible for the opening of the glottis, allowing for the production of high-pitched sounds. Damage to this muscle can lead to voice changes and breathing difficulties.

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