The correct answer is option 2, Sericin. Sericin is a protein that is found in silk fibers. It is produced by the silk worm during the production of silk threads. When the silk worm spins its cocoon, it produces sericin along with fibroin, which is the main protein that makes up the silk fibers.

Sericin plays a significant role in giving silk fibers their lustrous and shiny appearance. It forms a coating on the surface of the silk fibers, making them smooth and reflective. This smooth surface reflects light and gives silk its characteristic shine.

Fibrin, option 1, is a protein involved in blood clotting and is not related to silk production. Collagen, option 3, is a protein found in connective tissues like tendons and cartilage, and is not associated with silk fibers. Nectin, option 4, is a type of cell adhesion molecule and is also not related to silk fiber production.

In conclusion, the protein that gives a lustrous shiny appearance to silk fibers is sericin.