What primary substance are a bird's feathers and a human's fingernails composed of?

The primary substance that makes up a bird's feathers and a human's fingernails is a tough, fibrous protein.

A. Keratin is the correct answer. It is a structural protein that forms the main component of hair, skin, nails, feathers, horns, and claws in many animals.
B. Calcium is a mineral primarily found in bones and teeth.
C. Collagen is a protein found in connective tissues, skin, and bones, providing strength and elasticity.
D. Chitin is a polysaccharide found in the exoskeletons of insects and crustaceans, and in the cell walls of fungi.

The correct answer is A.

$$\boxed{\text{A. Keratin}}$$