**Core Concept**
The hypoglossal nerve is the twelfth cranial nerve (CN XII) responsible for controlling the motor functions of the tongue, including movement and protrusion. It is a mixed nerve that arises from the medulla oblongata in the brainstem and traverses the neck to innervate the extrinsic and intrinsic muscles of the tongue.

**Why the Correct Answer is Right**
The hypoglossal nerve is unique in that it is the only motor cranial nerve that originates from the brainstem and exits the cranium through the hypoglossal canal in the occipital bone. This nerve is a vital component of the tongue's motor function, enabling movements such as protrusion, retraction, and elevation. The nerve's motor fibers synapse with the motor nuclei of the tongue muscles in the medulla oblongata, allowing for coordinated movement of the tongue.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the trigeminal nerve (CN V) primarily controls facial sensation and some motor functions, but not tongue movement.
**Option B:** Incorrect because the glossopharyngeal nerve (CN IX) is involved in swallowing, taste, and some motor functions, but not tongue movement.
**Option C:** Incorrect because the vagus nerve (CN X) controls various functions, including swallowing, respiration, and heart rate, but not tongue movement.

**Clinical Pearl / High-Yield Fact**
The hypoglossal nerve is often tested in the clinical setting by asking the patient to protrude their tongue, which is a fundamental component of the cranial nerve examination.

**Correct Answer: A. Hypoglossal nerve.**