**Core Concept**
The epidermis is the outermost layer of the skin, composed of multiple layers of epithelial cells that provide a barrier against external factors. The epidermal layers vary in cell viability and function, with the outermost layer being non-viable.

**Why the Correct Answer is Right**
The stratum corneum, also known as the cornified layer, is the outermost layer of the epidermis. It is composed of fully differentiated, dead keratinocytes that have been flattened and compacted to form a tight barrier against water loss and external pathogens. The cells in this layer have undergone terminal differentiation and have lost their nuclei and most organelles, making them impermeable to water and other substances.

**Why Each Wrong Option is Incorrect**
**Option A:** Stratum basale - This is the innermost layer of the epidermis, composed of a single layer of actively dividing stem cells that produce new keratinocytes.

**Option B:** Stratum spinosum - This layer is composed of polyhedral cells with short spines, and is involved in the production of keratin filaments that provide mechanical strength to the skin.

**Option C:** Stratum granulosum - This layer is composed of cells that are undergoing the final stages of differentiation, and are characterized by the presence of granules that contain enzymes involved in the breakdown of cellular components.

**Clinical Pearl / High-Yield Fact**
The stratum corneum is responsible for the skin's barrier function, and is the primary site of water loss through the skin. It is also the site of action for many topical medications, such as creams and ointments.

**Correct Answer: C. Stratum corneum**