Silk is an:

Silk is an animal fiber and the strongest natural fiber.

Silk is obtained from the cocoon of silkworms.

Silk is an elastic, lustrous protein fiber.

Animal fibers- these are natural fibers which consist of large amounts of certain proteins.

Animal fibers include wool, hair and secretions, such as silk.

Plant fibres are obtained from plants.

Cotton fiber is one of the plant fiber.

Synthetic fibres are the man-made polymers designed to make a fabric. Polymers are obtained when many small units are joined together chemically. For example, rayon and nylon.

Hence, the correct option is A: i.e., animal fiber