**Core Concept**
The convergence of light rays on the retina is crucial for proper vision and is determined by the **refractive power** of the eye. The **cornea** and **lens** are the primary refractive structures, but one factor plays a dominant role in focusing light.

**Why the Correct Answer is Right**
The most important factor is the **cornea**, as it provides the majority of the eye's total refractive power due to its significant difference in refractive index between air and the cornea itself. The **lens** also contributes, especially in accommodation, but its role is secondary to the cornea's constant refractive power.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while important, it's not the most significant factor.
**Option B:** Not specified, but generally, other factors like the lens are less significant than the cornea.
**Option C:** Similarly, not the primary factor.
**Option D:** Not the most crucial factor in determining convergence.

**Clinical Pearl / High-Yield Fact**
Remember, the **cornea** is responsible for about 2/3 of the eye's total refractive power, making it crucial for vision. Understanding this helps in diagnosing and treating vision problems related to refractive errors.

**Correct Answer:** D. Cornea.