**Core Concept**
The question tests the understanding of **anatomical structures** of the eye, specifically where **cataract formation** occurs. Cataracts are characterized by the opacification of the **lens** in the eye, leading to impaired vision.

**Why the Correct Answer is Right**
Cataracts are formed due to the **denaturation of proteins** in the **lens** of the eye, leading to **opacification**. The **lens** is responsible for **focusing light** on the **retina**, and its opacification impairs this function, causing vision problems.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the **cornea** is the transparent outer layer of the eye, and while it can become opaque due to various conditions, cataracts specifically refer to opacifications in another structure.
**Option B:** This is incorrect as the **retina** is the innermost layer of the eye, responsible for converting light into neural signals, and is not where cataracts form.
**Option D:** This is incorrect because the **vitreous humor** is the gel-like substance filling the space between the **lens** and the **retina**, and although it can be involved in other types of eye disorders, cataracts are not formed here.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that cataracts are a very common cause of **vision loss** in older adults, and their formation is often related to **aging**, **diabetes**, or **trauma**.

**Correct Answer:** C. lens