Final answer:
The cortex is the part of the hair structure that contains melanin, giving hair its color. Melanin production varies over time, which can result in changes to hair color. The correct option is B.
Step-by-step explanation:
The structure that contains melanin to give hair its natural color is the cortex. The cortex is the middle layer of the hair, surrounded by the outer cuticle and potentially the inner medulla depending on the hair type. Melanin is the pigment responsible for the wide array of hair colors we see, produced by melanocytes located in the hair papilla.
The type of melanin present, which is genetically determined, results in different hair colors. Over time, melanin production can vary, leading to changes in hair color, such as the culmination of graying hair in older age.