**Core Concept**
The hypoglossal nerve is the twelfth cranial nerve, primarily responsible for controlling the motor functions of the tongue. It is a mixed nerve, containing both sensory and motor fibers, but its primary function is motor in nature.

**Why the Correct Answer is Right**
The hypoglossal nerve arises from the medulla oblongata and descends through the neck, eventually branching into the lingual nerve, which supplies the intrinsic and extrinsic muscles of the tongue. This nerve is essential for tongue movements, such as protrusion, retraction, and elevation. The motor function of the hypoglossal nerve is mediated by the hypoglossal nucleus, which is located in the medulla oblongata.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the trigeminal nerve (cranial nerve V) is primarily responsible for facial sensation and motor functions of the muscles of mastication.
* **Option B:** This option is incorrect because the vagus nerve (cranial nerve X) is a mixed nerve that controls various functions, including swallowing, vocal cord movement, and parasympathetic innervation of the thoracic and abdominal organs.
* **Option C:** This option is incorrect because the glossopharyngeal nerve (cranial nerve IX) is also a mixed nerve, primarily responsible for swallowing, taste sensation from the posterior one-third of the tongue, and parasympathetic innervation of the parotid gland.

**Clinical Pearl / High-Yield Fact**
The hypoglossal nerve is vulnerable to damage during surgical procedures in the neck, such as thyroidectomy or carotid endarterectomy. Damage to this nerve can result in weakness or paralysis of the tongue, leading to difficulties with speech and swallowing.

**Correct Answer: D. 12th cranial nerve responsible for motor functions of the tongue.**