**Core Concept**
The corneal stroma is the thickest layer of the cornea, comprising about 90% of its thickness. It is composed of highly organized collagen fibrils embedded in a proteoglycan-rich ground substance.

**Why the Correct Answer is Right**
The correct answer relates to the structural characteristics of the corneal stroma. The stroma is primarily composed of **type I collagen** fibrils, which are densely packed and arranged in a highly ordered manner to provide the cornea with its remarkable transparency and mechanical strength. The precise arrangement of these collagen fibrils and the ground substance is crucial for maintaining corneal clarity and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because the corneal stroma is not primarily composed of **type III collagen** fibrils, which are more commonly found in blood vessels and other connective tissues.
**Option B:** This statement is incorrect because the corneal stroma does not contain a significant amount of **elastin**, which is a protein that provides elasticity to tissues like skin and lungs.
**Option C:** This statement is incorrect because the corneal stroma is not primarily composed of **hyaluronic acid**, although it does contain some proteoglycans like keratan sulfate and chondroitin sulfate.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the corneal stroma's unique structure and composition are essential for maintaining corneal transparency and function. Damage to the stroma, such as that caused by keratoconus or corneal scars, can lead to significant visual impairment.

**Correct Answer:** D.