The correct answer is option 3, Pupa. The silkworm, also known as the silk moth larva, goes through four stages in its life cycle – egg, larva, pupa, and imago. The commercial use of silk is derived from the fiber produced by the silkworm during the pupa stage. During this stage, the silkworm spins a cocoon made of silk threads around itself. These threads are made of a protein called fibroin, which is secreted from the salivary glands of the silkworm larvae. The pupa stage is crucial in silk production because it is during this time that the silk thread is harvested by boiling the cocoon and unraveling the silk fibers. These fibers are then spun together to create silk fabric.