**Core Concept**
The normal eye power refers to the total optical power of the eye, which is the ability of the eye to converge light rays and focus them on the retina. This is a critical concept in ophthalmology and optometry, as it affects vision and the ability to see clearly.

**Why the Correct Answer is Right**
The normal eye power is approximately 60 diopters (D), which is the sum of the powers of the cornea (45 D), the lens (10 D), and the retina (5 D). The cornea provides the majority of the eye's optical power due to its steep curvature and high refractive index. The lens, which changes shape to focus on objects at different distances, also contributes to the eye's overall power. The retina, with its photoreceptors and neural layers, is responsible for detecting light and transmitting visual signals to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the normal eye power. The actual value is closer to 60 D, not 40 D.
**Option B:** This option is incorrect because it is too high and does not account for the contributions of the cornea, lens, and retina.
**Option C:** This option is incorrect because it is too low and does not accurately reflect the total optical power of the eye.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the normal eye power can vary slightly between individuals, but a value of approximately 60 D is a reliable estimate. This concept is crucial in understanding common vision problems, such as myopia (nearsightedness) and hyperopia (farsightedness), which occur when the eye's optical power is either too high or too low.

**Correct Answer: D. 60 diopters**