**Core Concept**
The question is testing the understanding of the **recurrent laryngeal nerve** and its function in relation to vocal cord movement. The **abducting function of the vocal cord** is primarily controlled by the **posterior cricoarytenoid muscle**, which is innervated by the **recurrent laryngeal nerve**.

**Why the Correct Answer is Right**
The **recurrent laryngeal nerve** is responsible for the innervation of all the intrinsic muscles of the larynx except the **crithyroid muscle**, which is innervated by the **external laryngeal nerve**. The **recurrent laryngeal nerve** controls the **abducting function** of the vocal cord through the **posterior cricoarytenoid muscle**.

**Why Each Wrong Option is Incorrect**
**Option A:** The **superior laryngeal nerve** is incorrect because it primarily controls the **crithyroid muscle** and is involved in tension of the vocal cords, not abduction.
**Option B:** The **vagus nerve** is incorrect because, while it is the main nerve from which the **recurrent laryngeal nerve** branches, it is not the specific nerve responsible for the **abducting function** of the vocal cord.
**Option D:** The **ansa cervicalis** is incorrect because it is a loop of nerves in the neck that supplies the **infrahyoid muscles**, not the muscles of the larynx.

**Clinical Pearl / High-Yield Fact**
The **recurrent laryngeal nerve** is at risk of injury in procedures involving the thyroid gland or thoracic surgery due to its proximity to these areas, and damage can result in **vocal cord paralysis**.

**Correct Answer:** D. Recurrent laryngeal nerve