Which among the following is a constituent of natural silk?

Natural silk is a protein fiber produced by silkworms (primarily Bombyx mori). It consists mainly of two proteins: fibroin, which forms the structural core, and sericin, the gummy substance that holds the fibers together.

Proteins are complex organic compounds made up of amino acids. Every amino acid contains four key elements: carbon, hydrogen, oxygen, and nitrogen. Therefore, nitrogen is a fundamental constituent of the chemical structure of silk. While trace minerals might be present depending on the silkworm's diet, phosphorus, magnesium, and potassium are not primary structural constituents of the silk protein itself.