**Core Concept**
The refractive power of the eye is determined by the combination of several components, including the cornea, lens, and vitreous humor. The maximum refractive power is contributed by the cornea, which is responsible for approximately two-thirds of the eye's total refractive power.

**Why the Correct Answer is Right**
The cornea's refractive power is due to its unique structure, which includes a steep curvature and a high refractive index. The cornea's surface is composed of a thin layer of epithelial cells, a thick layer of stroma, and a thin layer of endothelial cells. The stroma is the main contributor to the cornea's refractive power, as it contains a high concentration of collagen fibers that refract light. In addition, the cornea's anterior surface is more curved than the posterior surface, which further contributes to its refractive power.

**Why Each Wrong Option is Incorrect**
**Option A:** The lens, although responsible for accommodation and focusing, contributes only about one-third of the eye's total refractive power, making it a less significant contributor compared to the cornea.
**Option B:** The vitreous humor is a clear gel-like substance that fills the space between the lens and the retina, but it has a relatively low refractive index and therefore contributes minimally to the eye's refractive power.
**Option C:** The retina is a complex neural tissue that is responsible for converting light into electrical signals, but it does not contribute to the eye's refractive power.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cornea's refractive power is highly susceptible to changes in its curvature or refractive index, which can lead to vision problems such as myopia or hyperopia. Therefore, any condition that affects the cornea, such as keratoconus or corneal edema, can significantly impact the eye's refractive power.

**Correct Answer:** C. The cornea.