Right option is (a) Sulphide linkages between different amino acids
To elaborate: Primary structure of protein is formed by peptide linkages between two amino acids. Secondary structure, i.e. α-Helix and β-Sheath, of protein is formed by hydrogen bonds. Tertiary structure of protein is formed by sulphide linkages between different amino acids Quaternary structure is formed when multiple chains of polypeptides held together by hydrogen bonds.