The correct answer is option 4: Protein. Silk fibers are primarily composed of proteins, specifically a protein called fibroin. Fibroin is responsible for the strength and durability of silk fibers. It is made up of long chains of amino acids, which are the building blocks of proteins. These amino acids are arranged in a specific sequence that gives silk its unique properties.

Option 1: Carbohydrate is incorrect. Carbohydrates are not the main component of silk fibers. While silk does contain some small amounts of carbohydrates, they are not the predominant chemical composition.

Option 2: Complex lipid is incorrect. Lipids are not the primary component of silk fibers. While lipids may be present in small amounts, they do not make up the majority of the chemical composition of silk.

Option 3: Glycolipid is incorrect. Glycolipids are a specific type of lipid that contains a carbohydrate. While silk may contain some small amounts of glycolipids, they are not the main constituent of silk fibers.

Therefore, the correct answer is option 4: Protein. Silk fibers are predominantly made up of proteins, specifically the protein fibroin.