Explanation

Silk fibre consists of two proteins: fibroin, the structural core filament, and sericin, the gummy coating that binds the fibroin filaments together in the cocoon. Sericin makes up 25–30% of silk protein and is the protein conventionally credited in standard references with giving silk fibre its lustrous, shiny appearance. Fibrin is a blood-clotting protein (the option trades on confusion with 'fibroin'), collagen is the structural protein of connective tissue, and nectin is a cell-adhesion molecule — none of them occurs in silk. Among the options, therefore, sericin is the answer.