**Core Concept**
The question is testing the knowledge of the anatomical relationship between the acoustic neuroma and the surrounding cranial nerves. Acoustic neuroma, also known as vestibular schwannoma, is a benign tumor arising from the Schwann cells of the vestibulocochlear nerve (cranial nerve VIII). As the tumor grows, it can compress and involve adjacent cranial nerves, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The earliest involved cranial nerve is the facial nerve (cranial nerve VII). This is because the facial nerve is located adjacent to the vestibulocochlear nerve and is more susceptible to compression by the growing tumor. The facial nerve is responsible for controlling the muscles of facial expression, and early involvement can lead to symptoms such as facial weakness or numbness. The close proximity of the facial nerve to the acoustic neuroma makes it the first cranial nerve to be affected.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the trigeminal nerve (cranial nerve V) is not the earliest involved cranial nerve. While it can be affected later in the course of the disease, the facial nerve is more susceptible to compression due to its anatomical relationship with the acoustic neuroma.

**Option B:** This option is incorrect because the glossopharyngeal nerve (cranial nerve IX) is not typically involved early in the course of acoustic neuroma. The glossopharyngeal nerve is located more medially and is not as closely associated with the vestibulocochlear nerve.

**Option C:** This option is incorrect because the vagus nerve (cranial nerve X) is not the earliest involved cranial nerve. While it can be affected later in the course of the disease, the facial nerve is more susceptible to compression due to its anatomical relationship with the acoustic neuroma.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that the facial nerve is often the first to be affected in acoustic neuroma, leading to symptoms such as facial weakness or numbness. This is due to the close proximity of the facial nerve to the vestibulocochlear nerve and the tumor.

**Correct Answer:** A. Facial nerve.