Silkworms go through four stages in their life-cycle: egg, larva, pupa, and imago. The term `imago` refers to the adult stage, whereas `larva` and `pupa` denote the developmental phases.

Option 1: `Egg` is the initial stage of the silkworm lifecycle. At this point, no silk is produced.

Option 2: `Larva` is when the silkworm hatches and keeps eating mulberry leaves. While it is during this stage the silkworm produces the silk, it is not harvested for use.

Option 3: `Pupa` is the correct answer. The silkworm wraps itself in a cocoon made of silk threads before it enters this phase - it is this silk thread that is the fiber of commerce.

Option 4: `Imago` is the adult stage of the silkworm. By this point, the silk has already been produced and would have been harvested if it was in a silk farm.

Therefore, while the silkworm produces silk in the larval stage, it`s only harvested once they enter the pupa phase making `pupa` the correct answer.