At which stage in its life-cycle does the silkworm yield the fibre of commerce?

The correct answer is Option 3 (Pupa).

The life cycle of a silkworm (Bombyx mori) consists of four stages: Egg, Larva (Caterpillar), Pupa (Chrysalis), and Imago (Adult moth). The silk fiber of commerce is obtained during the pupa stage.

• Larva: During the final phase of the larval stage, the caterpillar secretes liquid silk from its salivary glands to spin a protective protective shell called a cocoon.
• Pupa: Once the cocoon is complete, the larva transforms into a pupa inside. It is this cocoon, harvested while the insect is in the pupal stage, that yields the continuous silk filament.
• Imago: If the pupa is allowed to mature into an adult (imago), it secretes a fluid to dissolve the silk to emerge, thereby breaking the long continuous fibers and reducing their commercial value.

Therefore, to maintain the quality and length of the fiber, the silk is extracted at the pupal stage.