Which among the following is a constituent of natural silk?

1. A

   Phosphorus

   Option (a) claims that phosphorus is a constituent of natural silk. However, phosphorus is primarily associated with nucleic acids and bones rather than the protein structure of silk fibres.

2. B

   Nitrogen

   Option (b) claims that nitrogen is a constituent of natural silk, which is correct because natural silk is a protein fibre composed of amino acids such as fibroin and sericin, both of which contain nitrogen as a fundamental element of their peptide backbone. Therefore, this option accurately identifies the chemical component.

3. C

   Magnesium

   Option (c) claims that magnesium is a constituent of natural silk. Magnesium is typically found in chlorophyll and various enzyme cofactors rather than being a primary building block of animal silk proteins.

4. D

   Potassium

   Option (d) claims that potassium is a constituent of natural silk. Potassium is an essential macronutrient for plants and an intracellular cation in animals, but it is not part of the structural amino acid framework of silk.

Summary. Official key is (b) Nitrogen. Natural silk is an animal protein produced by silkworms, primarily consisting of fibroin, which is rich in amino acids containing carbon, hydrogen, oxygen, and nitrogen. While other elements may be present in trace amounts from the diet of the silkworm, nitrogen forms the core peptide bonds of the protein chains. Thus, nitrogen is correctly identified as a primary constituent among the given choices.