Silk is obtained from the cocoon of a silk worm. The process of obtaining silk begins when a silk worm spins a protective cocoon around itself. The cocoon is made up of a single thread of silk, which can be up to 900 meters long. Once the cocoons are harvested, they are boiled to kill the silk worms inside and loosen the silk fibers. The fibers are then unwound from the cocoon and spun into silk threads. It is important to note that the silk worm larvae produce the silk, but it is specifically the cocoon that is harvested to obtain silk. Therefore, option 4 is the correct answer as it accurately identifies the source of silk.