Final answer:
The hair structure that serves as a storage for stem cells is the outer root sheath.
Step-by-step explanation:
The correct answer to which hair structure serves as a storage for stem cells is D) outer root sheath. The hair follicle has various structures and layers, including the hair matrix and the inner root sheath. The hair matrix contains a layer of basal cells from which a strand of hair grows.
However, it is the outer root sheath, an extension of the epidermis that runs along the hair follicle, that houses the stem cells capable of regenerating the hair follicle as well as contributing to the healing of the epidermis.