**Core Concept**
The question is referring to the structure of the foot, specifically the layers of the plantar surface of the foot. The plantar surface of the foot is composed of three layers: the epidermis, dermis, and hypodermis. The first layer of the sole, also known as the epidermal layer, is the outermost layer of the plantar surface.

**Why the Correct Answer is Right**
The epidermal layer is the first layer of the sole, and it is composed of stratified squamous epithelial cells that provide a protective barrier against external factors such as friction, pressure, and moisture. This layer is rich in keratin, a protein that provides strength and rigidity to the skin. The epidermal layer also contains melanocytes, which produce melanin, the pigment responsible for skin color.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a recognized layer of the plantar surface of the foot.
* **Option B:** This option is incorrect because it is not the first layer of the sole; it is actually one of the deeper layers.
* **Option C:** This option is incorrect because it is not a specific layer of the plantar surface of the foot.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the plantar surface of the foot is subjected to significant pressure and friction, which can lead to the development of plantar fasciitis, a common condition characterized by inflammation of the plantar fascia, a band of tissue that runs along the bottom of the foot.

**Correct Answer: D. Epidermal layer of the skin.**