Which part of the hair consists of hard keratinized flat spindle-shaped cells containing pigment?

The cortex is the correct answer because it is the part of the hair that contains the bulk of the hair's structure. It is composed of hard keratinized flat spindle-shaped cells and contains pigment, which is responsible for the hair color. The cortex lies beneath the cuticle and surrounds the medulla, contributing to the hair's strength and overall integrity.

In contrast, the medulla is the innermost layer of the hair shaft and may contain air spaces, but it generally doesn't play a significant role in pigment or structural integrity. The cuticle, consisting of overlapping flat cells, serves mainly as a protective layer for the cortex and does not contribute to the hair's color or the keratinization process. The follicle, while crucial for hair growth, is not part of the hair shaft itself and does not contain the keratinized cells that make up the structure of the hair. Hence, the cortex is essential for its role in providing strength and pigmentation to hair.