Concept:Silkworms produce silk fibre from a protein secretion that hardens on contact with air.Explanation:The caterpillar (silkworm) secretes a fluid made of protein.This fluid hardens into silk fibre when exposed to air.The silkworm wraps itself in these fibres to form a cocoon.Later, the cocoon is processed to extract silk threads.Silk is a natural protein fibre, not a carbohydrate, fat, or cellulose.Answer:A. Proteins