**Core Concept**
The sensory nerve supply to the larynx below the level of the vocal cord involves the innervation of the mucosa lining the laryngeal structure. This includes the epiglottis, aryepiglottic fold, arytenoid cartilage, and the posterior and lateral walls of the larynx.

**Why the Correct Answer is Right**
The recurrent laryngeal nerve (C) is responsible for the sensory innervation of the larynx below the vocal cord level. It arises from the vagus nerve (cranial nerve X) and descends in the neck, eventually passing behind the subclavian artery (on the right) or behind the ligamentum arteriosum (on the left). The recurrent laryngeal nerve then ascends to innervate the intrinsic muscles of the larynx and the sensory receptors in the laryngeal mucosa below the level of the vocal cord. This includes the cricoid cartilage, the posterior and lateral walls of the larynx, and the arytenoid cartilages.

**Why Each Wrong Option is Incorrect**
**Option A:** The external branch of the superior laryngeal nerve primarily supplies the cricothyroid muscle, which is involved in adjusting the tension of the vocal cords, but it does not provide sensory innervation to the larynx below the level of the vocal cord.

**Option B:** The internal branch of the superior laryngeal nerve provides sensory innervation to the mucosa of the larynx above the level of the vocal cord, specifically the area between the epiglottis and the vocal cords.

**Option D:** The inferior pharyngeal nerve is not directly involved in the sensory innervation of the larynx. It primarily supplies the pharyngeal mucosa and the stylopharyngeus muscle.

**Clinical Pearl / High-Yield Fact**
Remember that the recurrent laryngeal nerve has a long course in the neck, which makes it susceptible to injury during thyroid and parathyroid surgery. This can result in vocal cord paralysis, leading to hoarseness of voice or difficulty swallowing.

**Correct Answer:**
✓ Correct Answer: C. Recurrent laryngeal nerve