Question

what is the function of the protein keratin?
to provide support and structure for cells in hair, skin, and nails
to fight infections caused by bacteria and viruses
to increase chemical reaction rates in the digestive system
to transmit chemical signals throughout the body

Explanation:

Keratin is a fibrous structural protein. In hair, skin, and nails, it forms a tough, protective layer. Antibodies (not keratin) fight infections. Enzymes increase chemical reaction rates. Hormones (not keratin) transmit chemical signals.

Answer:

to provide support and structure for cells in hair, skin, and nails