To solve this problem, we need to identify the type of bond that links the two polypeptides of human insulin. Let's analyze the options:$\newline \bullet$Option 1: Hydrogen bonds$\newline$ - Hydrogen bonds are weak interactions that occur between a hydrogen atom covalently bonded to a more electronegative atom and another electronegative atom.$\newline$ - They are not responsible for linking polypeptides in insulin.$\newline \bullet$Option 2: Phosphodiester bond$\newline$ - Phosphodiester bonds are found in nucleic acids, linking nucleotides together.$\newline$ - They are not involved in linking polypeptides in proteins like insulin.$\newline \bullet$Option 3: Covalent bond$\newline$ - Covalent bonds involve the sharing of electron pairs between atoms.$\newline$ - While disulphide bridges are a type of covalent bond, this option is too general.$\newline \bullet$Option 4: Disulphide bridges$\newline$ - Disulphide bridges are covalent bonds specifically formed between the sulfur atoms of two cysteine residues.$\newline$ - In insulin, these bridges link the A and B polypeptide chains together, stabilizing the structure.$\newline$Therefore, the correct answer is Option 4: Disulphide bridges.