Proteins are found to have two different types of secondary structures viz. α-helix and β-pleated sheet structure. α-helix structure of protein is stabilized by Hydrogen bonds. This hydrogen bonds are formed between backbone N-H group of an amino acid with the backbone C=O group of every fourth amino acid in the peptide bond. Thus, option (iii) is correct.

Option (i) is incorrect because peptide bonds are present between the amino acids.

Options (ii) and (iv) are incorrect because α-helix structure of protein is mainly stabilized by Hydrogen bonds and not by van der Waals forces or Dipole-dipole interactions.