Final answer:
The hair follicle that produces tightly curled hair is flattened in an elliptical shape.
Step-by-step explanation:
The hair follicle that produces tightly curled hair is d) elliptical shape.
Cell shapes are often dominated by tension, and the shape of the hair follicle is determined by the shape of the cells in the follicle. In the reference information provided, it suggests that cells can be round, invaginated, polygonal, or cylindrical. The elliptical shape of the hair follicle aligns with the shape of a cylindrical cell extension like an axon, which tends to pearl under increased tension. Therefore, the hair follicle that produces tightly curled hair is most likely flattened in an elliptical shape.