Mitotic cells in the hair begin their growth activity in which part?

The correct choice is the matrix, which plays a crucial role in the hair growth process. The hair matrix is located at the base of the hair follicle, specifically in a part known as the hair bulb. This area is where the mitotic cells (those that actively divide) reside and are responsible for producing new hair cells. As these cells divide, they push older cells upwards, eventually leading to the formation of the hair shaft above the surface of the skin.

Understanding the function of the matrix is essential because it is the active site of hair production. The cells in this area are continuously undergoing mitosis, enabling hair to grow. This cellular activity is crucial for both the growth and regeneration of hair strands, making the matrix integral to the overall hair growth process. The other parts mentioned, while related to hair structure and function, do not specifically host the primary activity of cell division leading to new hair growth.