Each polypeptide A. primary structure of proteins.

B. secondary structure of proteins.

C. tertiary structure of proteins.

D. quaternary structure of proteins.

Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be primary structure of proteins. Here, amino acids are joined together by a bond called peptide bond or peptide linkage. Thus, option (i) is correct.

Option (ii) is incorrect because primary structure of proteins is then coiled into secondary structure by the hydrogen bonding in the backbone of the peptides.

Option (iii) is incorrect because tertiary structure of proteins represents the overall folding of the polypeptide chains.

Option (iv) is incorrect because quaternary structure of proteins are composed of two or more polypeptide chains.