Step 1: Understanding the Concept:
The question asks to identify the natural fiber that is sourced from the protective casing spun by an insect larva.
Step 2: Detailed Explanation:
- Silk is a natural protein fiber produced by the larva of certain insects, most notably the mulberry silkworm (Bombyx mori), to form a cocoon. The cocoon is then harvested and processed to extract the silk fiber.
- Wool is a fiber obtained from the fleece of sheep and other animals.
- Linen is a plant fiber obtained from the flax plant.
- Cotton is a plant fiber obtained from the bolls of the cotton plant.
Step 3: Final Answer:
Silk is the fiber obtained from the cocoon of a silkworm.