## Core Concept
The question tests knowledge of cerebral circulation, specifically the formation of major arteries from the vertebral arteries. The **vertebral arteries** are two significant arteries that arise from the subclavian arteries and travel through the neck to supply blood to the brain.

## Why the Correct Answer is Right
The **basilar artery** is formed by the fusion of the two vertebral arteries. This occurs at the base of the brain, near the junction of the pons and the medulla oblongata. The basilar artery then continues upward to supply blood to the brainstem, cerebellum, and posterior parts of the brain. It eventually bifurcates into the two posterior cerebral arteries.

## Why Each Wrong Option is Incorrect
* **Option A:** The internal carotid artery is a major artery supplying blood to the brain but is not formed by the joining of the vertebral arteries; rather, it branches off from the common carotid artery.
* **Option B:** The anterior cerebral artery is a branch of the internal carotid artery and is not formed by the vertebral arteries.
* **Option D:** The posterior cerebral artery is a branch of the basilar artery, not the direct result of the joining of the vertebral arteries.

## Clinical Pearl / High-Yield Fact
A key point to remember is the "vertebral-basilar" system, which includes the vertebral arteries joining to form the **basilar artery**. This system is crucial for supplying blood to the posterior circulation of the brain, including the brainstem and cerebellum.

## Correct Answer: C. Basilar artery