**Core Concept**
The sensation of the posterior aspect of the tongue is mediated by a specific cranial nerve, which is responsible for transmitting sensory information from this region. This nerve is distinct from those providing motor innervation or sensation to other parts of the tongue.

**Why the Correct Answer is Right**
The glossopharyngeal nerve (cranial nerve IX) is responsible for the sensation of the posterior one-third of the tongue. This nerve provides sensory innervation to the mucous membrane of the posterior tongue, including the circumvallate papillae, where taste buds are found. The glossopharyngeal nerve also carries taste fibers from the posterior tongue to the solitary nucleus in the medulla oblongata. Additionally, it provides parasympathetic innervation to the parotid gland and motor innervation to the stylopharyngeus muscle.

**Why Each Wrong Option is Incorrect**
**Option B:** The vagus nerve (cranial nerve X) provides sensation to the epiglottis, the walls and floor of the pharynx, and the larynx, but not the posterior aspect of the tongue.
**Option C:** The hypoglossal nerve (cranial nerve XII) is a motor nerve that innervates the muscles of the tongue, but it does not provide sensation.
**Option D:** The mandibular nerve (a branch of the trigeminal nerve, cranial nerve V) provides sensation to the lower face and the anterior two-thirds of the tongue, but not the posterior aspect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sensation of the posterior tongue is unique to the glossopharyngeal nerve, and any damage to this nerve can result in loss of taste and sensation in this region. This is crucial for patients undergoing surgery in the neck or those with conditions affecting the glossopharyngeal nerve.

**✓ Correct Answer: A. Glossopharyngeal nerve**