## Core Concept
The circle of Willis is a critical structure in cerebral circulation. It is an anastomotic circle of arteries that supplies blood to the brain and surrounding structures. It is located at the base of the brain.

## Why the Correct Answer is Right
The circle of Willis is situated in the subarachnoid space at the base of the brain, specifically around the optic chiasm and the pituitary stalk. It is formed by the internal carotid arteries and the anterior and posterior cerebral arteries, along with the anterior and posterior communicating arteries. This circle provides a safety mechanism to ensure a constant blood supply to the brain, even if one of the arteries becomes occluded.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately describe the location of the circle of Willis.
* **Option B:** This option is incorrect because it does not accurately describe the location of the circle of Willis.
* **Option D:** This option is incorrect because it does not accurately describe the location of the circle of Willis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the circle of Willis is a common site for aneurysms, which can rupture and cause subarachnoid hemorrhage. Understanding the anatomy of the circle of Willis is crucial for diagnosing and managing cerebrovascular diseases.

## Correct Answer Line
**Correct Answer: C. Base of brain.**