**Core Concept**
The refractive power of the eye is determined by the curvature of the cornea and the lens, as well as the length of the eye. Emmetropia is a state of normal vision, where the eye is able to focus light correctly on the retina without the need for corrective lenses.

**Why the Correct Answer is Right**
The refractive power of an emmetropic eye is approximately 60 diopters (D). This is because the eye has a balanced amount of corneal and lenticular refractive power, allowing it to focus light correctly on the retina. The cornea contributes approximately 40 D, while the lens contributes around 20 D. The eye's axial length is also optimized to ensure that the focal point of light falls on the retina.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a refractive power of 20 D is too low to be considered emmetropic.
* **Option B:** This option is incorrect because a refractive power of 80 D is too high to be considered emmetropic.
* **Option D:** This option is incorrect because it is not a specific value and does not accurately represent the refractive power of an emmetropic eye.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that emmetropia is a dynamic state, and the refractive power of the eye can change over time due to factors such as aging, myopia, or hyperopia.

**Correct Answer: C. 60 diopters.**