**Core Concept**
The abductor of the vocal cord is responsible for opening the glottis, enabling airflow during inhalation. This action is crucial for maintaining normal respiratory function. The abductor muscle is innervated by the recurrent laryngeal nerve, a branch of the vagus nerve.

**Why the Correct Answer is Right**
The correct muscle responsible for abducting the vocal cord is the posterior cricoarytenoid muscle. This muscle acts by contracting the arytenoid cartilage, which in turn rotates the vocal process laterally, thus opening the glottis. The posterior cricoarytenoid muscle is the only abductor of the vocal cord, making it essential for maintaining normal respiratory function.

**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is primarily an adductor of the vocal cord, not an abductor. It helps in closing the glottis during phonation.
* **Option B:** The lateral cricoarytenoid muscle is a secondary adductor of the vocal cord. It assists in closing the glottis during phonation, but not in opening it.
* **Option D:** The cricothyroid muscle is involved in tensing the vocal cords, but it does not abduct the vocal cords.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only abductor of the vocal cord, making it essential to consider in cases of respiratory distress or difficulty with inhalation.

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