Final answer:
A wiglet is the hair piece made up of hair fibers of six inches or less in length attached to a flat base, and the hair matrix contains a layer of basal cells.
Step-by-step explanation:
The hair piece that is made up of hair fibers six inches or less in length attached to a round, flat base is known as a wiglet. A wiglet can be used to add volume to existing hair and is distinct from other hair pieces such as toupees, which are typically used to cover bald spots, or hair extensions, which add length to one's natural hair.
The hair matrix mentioned in the reference contains a layer of basal cells, which is the correct answer to the question about the hair matrix's content. These basal cells are responsible for the production of new hair cells, leading to hair growth.