## Core Concept
The corneal endothelium is a single layer of cells on the inner surface of the cornea, crucial for maintaining corneal transparency by regulating its hydration. The endothelial cell density is a key factor in assessing the health and function of the corneal endothelium.

## Why the Correct Answer is Right
The normal endothelial cell density in a human cornea is generally considered to be around 1,500 to 2,500 cells per square millimeter (cells/mm²). This density is essential for the endothelium to effectively pump out excess fluid from the cornea, thereby maintaining its clarity. A cell density within this range ensures that the cornea remains dehydrated and transparent, which is vital for clear vision.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a range or a specific value that falls within the known normal range for endothelial cell density.
- **Option B:** Similarly, this option is incorrect for the same reason as Option A; it lacks a specific value or range that matches the established norms for endothelial cell density.
- **Option D:** This option suggests a much higher or possibly unrelated value (as the actual value is not provided) that does not align with the established normal range for endothelial cell density.

## Clinical Pearl / High-Yield Fact
A significant clinical pearl is that the endothelial cell density decreases with age and can be affected by various factors such as ocular surgery, trauma, and certain diseases. A minimum endothelial cell density is required to maintain corneal transparency, and values significantly below this threshold can lead to corneal edema and vision problems.

## Correct Answer: C. 2000/mm².