The correct answer is option 1: Protein. Chemically, silk fibers are predominantly made up of proteins. Proteins are large molecules made up of amino acids, and they are essential for the structure and function of living organisms. Silk is produced by certain insects, such as silkworms, to construct their cocoons.

Silk fibers are composed of two proteins called fibroin and sericin. Fibroin makes up the majority of the silk fiber and provides strength and flexibility. It forms a long, continuous chain of amino acids that are arranged in a structure called a beta-sheet. This arrangement gives silk its characteristic smoothness and luster.

Sericin is a sticky protein that surrounds the fibroin fibers within the silk cocoon. It acts as a protective layer and helps bind the fibers together. Sericin is removed during the process of extracting silk from the cocoon to obtain the pure fibroin fibers.

Therefore, chemically, silk fibres are predominantly protein-based, specifically composed of fibroin and sericin proteins.