**Core Concept**
Unilateral trigeminal nerve injury can lead to ipsilateral facial sensory loss and motor weakness, affecting the muscles of mastication. The trigeminal nerve has three main divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3), each responsible for different facial sensory and motor functions.

**Why the Correct Answer is Right**
The correct answer is a test that specifically assesses the function of the trigeminal nerve. The corneal reflex, also known as the blink reflex, is a crucial test for evaluating the integrity of the trigeminal nerve. This reflex involves the afferent pathway of the trigeminal nerve (sensory input from the cornea) and the efferent pathway of the facial nerve (motor response of the orbicularis oculi muscle). When the cornea is stimulated, the afferent signal is transmitted to the trigeminal nerve, which then sends a signal to the facial nerve, resulting in a blink response. This reflex is essential for assessing trigeminal nerve function, particularly in cases of suspected nerve injury or compression.

**Why Each Wrong Option is Incorrect**
* **Option A:** The gag reflex is primarily mediated by the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X), making it an inappropriate test for trigeminal nerve function.
* **Option B:** The jaw jerk reflex is a monosynaptic reflex involving the trigeminal nerve but is not as sensitive or specific as the corneal reflex for assessing trigeminal nerve function.
* **Option C:** The oculovestibular reflex involves the vestibulocochlear nerve (cranial nerve VIII) and the facial nerve (cranial nerve VII), making it an inappropriate test for trigeminal nerve function.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that the corneal reflex is an essential test for evaluating trigeminal nerve function, particularly in cases of suspected trigeminal neuralgia or compression. A reduced or absent corneal reflex can indicate trigeminal nerve damage.

**Correct Answer: C. Jaw jerk reflex.**