**Core Concept**
The cornea is composed of five distinct layers, each with unique properties and functions. The metabolic activity of the cornea is primarily attributed to the epithelial layer, which is responsible for maintaining corneal health and transparency.

**Why the Correct Answer is Right**
The corneal epithelium is metabolically active due to the presence of high concentrations of mitochondria and a rich network of blood vessels. This layer is responsible for the production of enzymes, growth factors, and other molecules that maintain corneal homeostasis. The corneal epithelium also contains sensory nerve endings that provide sensation to the cornea, making it an essential layer for maintaining corneal health.

**Why Each Wrong Option is Incorrect**
**Option A:** The Bowman's layer is a dense, acellular layer composed of collagen fibers and is not metabolically active.
**Option B:** The stroma is the thickest layer of the cornea, composed of collagen and keratocytes, but it is not as metabolically active as the epithelial layer.
**Option C:** The endothelium is a single layer of cells that lines the posterior surface of the cornea and is responsible for maintaining corneal deturgescence, but it is not as metabolically active as the epithelial layer.

**Clinical Pearl / High-Yield Fact**
The corneal epithelium is responsible for the production of mucins, which play a crucial role in maintaining corneal surface health and preventing bacterial adhesion.

**Correct Answer: C. Stroma**