The two polypeptides of human insulin are linked together by

The A peptide and B peptide of human insulin are linked together by disulphide bridges, which are strong covalent bonds formed between the sulphur atoms in cysteine residues on each polypeptide chain. Mature insulin has two interchain disulphide bonds, one between the amino acids A7 and B7 and the other between the amino acids A20
and B19. Mature insulin is also an intrachain disulphide bridge between the amino acids A6 and A11.

(1) Covalent bonds can refer to a variety of chemical bonds in which atoms share electrons, but the specific type of covalent bond that links the two polypeptides of insulin together is a disulphide bond, not any type of covalent bond.
(3) Hydrogen bonds are weaker and do not link the A and B chains of insulin together.
(4) Phosphodiester bonds link nucleotides together in DNA and RNA molecules.