**Core Concept**
The question is testing the knowledge of the muscles involved in the larynx (voice box) and their functions. The abductor of the larynx is responsible for opening the glottis, which is the space between the vocal folds. This muscle plays a crucial role in breathing and preventing asphyxiation.

**Why the Correct Answer is Right**
The correct answer is the posterior cricoarytenoid muscle. This muscle is innervated by the recurrent laryngeal nerve, a branch of the vagus nerve (cranial nerve X). The posterior cricoarytenoid muscle acts by rotating the arytenoid cartilages, which in turn opens the glottis and allows for inhalation of air. This muscle is the only abductor of the vocal folds, and its paralysis can lead to respiratory distress.

**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is an adductor of the vocal folds, not an abductor. It helps to close the glottis and produce sound.
* **Option B:** The cricothyroid muscle is involved in tensing the vocal folds and altering pitch, but it is not an abductor of the glottis.
* **Option C:** The arytenoid muscle is involved in adduction and closure of the glottis, not abduction.

**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only muscle that opens the glottis, and its paralysis can lead to respiratory distress. This is a classic exam trap, as students may mistakenly think that other muscles are involved in abduction of the vocal folds.

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