Final answer:
The correct answer is c) density, which indicates the number of active hair follicles per square inch on the scalp.
Step-by-step explanation:
The number of active hair follicles per square inch on the scalp is a measurement of hair density. Hair density is an important aspect of hair that affects its volume and the way it styles. It is not related to hair color, which is determined by the types of melanin (eumelanin and pheomelanin) present in the hair follicles, nor is it related to hair texture which includes characteristics such as curl pattern, thickness, and consistency. Porosity, on the other hand, refers to the hair's ability to absorb and retain moisture.