Final answer:
The correct option is a. A hair follicle has a conical shape, with the hair matrix located at its base containing a layer of basal cells. Cell shapes in biology are diverse and often depend on their environment and function.
Step-by-step explanation:
The shape of a hair follicle is generally conical. It is not circular, spiral, or rectangular, as the follicle tapers from the wider base at the skin's surface to a narrowed end deeper in the skin. Within the hair follicle, the hair matrix is a crucial structure consisting of rapidly dividing cells, which give rise to the hair shaft. The matrix contains a layer of basal cells and is located at the base of the hair follicle.
The shape and behavior of cells are influenced by various factors; for instance, isolated cells such as fibroblasts may have an invaginated shape, while epithelial cells tend to be polygonal. In a broader sense, the shapes of biological structures can be quite diverse, including cylindrical cell extensions like axons and the helical structures of certain proteins.