What is the part of the hair contained within the follicle below the surface of the scalp?

The hair root is the correct answer because it refers specifically to the part of the hair that is embedded within the follicle beneath the surface of the scalp. This section of the hair is not visible and is responsible for growth. It includes the lower part of the hair strand that is anchored in the dermal papilla, where the blood supply and nerve endings are located, facilitating nourishment and hair growth.

In contrast, the hair follicle is the structure in which the root resides but encompasses more than just the root itself; it includes the surrounding tissue that supports and grows the hair. The hair bulb is the rounded club-shaped structure at the base of the hair root, where the hair grows, but it is a part of the root rather than synonymous with it. The hair shaft is the visible part of the hair that extends above the scalp and is not contained within the follicle, making it distinct from the root.