What is tortoise shell primarily composed of?

Tortoise shell is primarily composed of a type of protein known as keratin, which makes option B the correct choice. Keratin is a fibrous structural protein found in various biological materials, including skin, hair, nails, and the shells of certain animals such as turtles. The unique structure of keratin contributes to the tough and durable nature of tortoise shell, which has historically been valued for its aesthetic qualities and used in a variety of decorative items and accessories.

While cellulose, calcium, and wax are important substances in other biological contexts, they do not form the primary component of tortoise shell. Cellulose is a polysaccharide that provides structural support in plants, calcium is an essential mineral that forms bones and shells in various organisms, and wax is a lipid that serves various protective and waterproofing roles in nature. Each of these materials has distinct characteristics and functions that differ from those of keratin, which is why they do not apply in this context regarding the composition of tortoise shell.