**Core Concept**
The refractive index of a medium is a measure of how much it bends light. In the context of the eye, the refractive index determines the medium's ability to focus light. The eye's refractive index varies between different components, such as the cornea, aqueous humor, lens, and vitreous humor.

**Why the Correct Answer is Right**
The correct answer is the lens. The lens has a high refractive index, typically around 1.406, due to its high concentration of proteins and lipids. This high refractive index allows the lens to focus light and change its shape to accommodate objects at different distances. The lens's refractive index is crucial for the eye's ability to focus and is a key factor in visual acuity.

**Why Each Wrong Option is Incorrect**
* **Option A:** The cornea has a refractive index of around 1.376, which is lower than the lens. Although the cornea contributes significantly to the eye's total refractive power, its refractive index is not the highest.
* **Option B:** The aqueous humor has a refractive index of around 1.336, which is lower than both the cornea and the lens. The aqueous humor's refractive index is relatively low compared to other components of the eye.
* **Option D:** The vitreous humor has a refractive index of around 1.336, which is similar to the aqueous humor. The vitreous humor's refractive index is also relatively low compared to the lens.

**Clinical Pearl / High-Yield Fact**
The lens's high refractive index is essential for the eye's ability to focus light and change its shape to accommodate objects at different distances. This is why the lens is often referred to as the eye's "variable focus" component.

**Correct Answer:** C. Lens.