Option 1: Nylon is a synthetic fiber and is not a protein fiber. It is made from petrochemicals and does not have protein as its main component.
Option 2: Polyester is also a synthetic fiber, made from petrochemicals. It does not contain protein and is not a protein fiber.
Option 3: Silk is the correct answer. Silk is a natural fiber and is made from proteins, specifically fibroin. It is produced by certain insects, such as silkworms, and is known for its lustrous appearance and smooth feel.
Option 4: Cotton is a natural fiber but it is not a protein fiber. It is made from cellulose, a carbohydrate, and is not composed of proteins.
Therefore, the correct answer is option 3, Silk. It is the only option that is a protein fiber.