What is a small sac at the root of any strand hair?

The correct answer is the Hair Follicle.

The hair follicle is a small, tube-shaped organ that produces hair. It is located in the skin and contains the hair shaft, hair root, and sebaceous gland. The hair shaft is the visible part of the hair, while the hair root is the part that is embedded in the skin. The sebaceous gland produces an oily substance called sebum, which helps to keep the hair healthy and shiny.