Final answer:
The central core of the hair shaft, often absent in fine hair, is called the medulla. The medulla is surrounded by the cortex and covered by the cuticle. The correct answer is d) medulla.
Step-by-step explanation:
The central core of the hair shaft, which is often absent in fine or very fine hair, is called the medulla. A hair shaft consists of three zones: the cuticle, the cortex, and the medulla. The cuticle is the outermost layer, while the cortex is the middle layer that determines the hair's texture (straight, curly) and is composed of compressed, keratinized cells. The medulla, when present, forms the innermost core. In the context of the provided options, the correct answer is d) medulla.
The hair matrix contains a layer of basal cells that are responsible for the production of keratinocytes, which are the cells that form the hair shaft as they undergo keratinization. The hair matrix is the layer from which a strand of hair grows.