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.