**Core Concept**
Acoustic neuromas, also known as vestibular schwannomas, are benign tumors arising from the Schwann cells of the vestibulocochlear nerve (CN VIII). These tumors can cause compression of surrounding structures, leading to various neurological deficits.

**Why the Correct Answer is Right**
The earliest cranial nerve involved in acoustic neuromas is the trigeminal nerve (CN V), specifically its fifth cranial nerve branch, the trigeminal nerve's sensory root. This is because the tumor typically arises from the Schwann cells of the vestibulocochlear nerve, which is closely associated with the trigeminal nerve's sensory root. Compression of the trigeminal nerve's sensory root can lead to symptoms such as facial numbness, specifically affecting the areas supplied by the ophthalmic and maxillary branches of the trigeminal nerve.

**Why Each Wrong Option is Incorrect**
* **Option A:** The facial nerve (CN VII) is often involved later in the course of acoustic neuroma, as the tumor grows and compresses this nerve, leading to symptoms such as facial weakness or paralysis.
* **Option B:** The glossopharyngeal nerve (CN IX) is not typically involved in acoustic neuromas, as it is located in a different anatomical location.
* **Option D:** The vagus nerve (CN X) is also not typically involved in acoustic neuromas, as it is located in a different anatomical location.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that acoustic neuromas can cause subtle symptoms, such as facial numbness, which may precede more obvious symptoms, such as hearing loss or balance disturbances.

**Correct Answer: C. Trigeminal nerve (CN V)**