Proteins are stabilized by diverse bonds that connect parts of the polypeptide chains. Hydrogen bonds form between polar groups, such as the backbone carbonyl and amide groups, and help hold secondary structures like alpha helices and beta sheets in place and can also link distant regions of a folded protein. Salt bridges are ionic interactions between oppositely charged side chains (for example, lysine or arginine with aspartate or glutamate). These ionic contacts can tie different regions together, contributing to the stability of tertiary and sometimes quaternary structure. Disulfide bonds are covalent links formed between cysteine residues; they can connect separate polypeptide chains or link distant parts of the same chain, providing strong cross-links that help lock in a protein’s final shape. Because each of these types of bonds can cross-link polypeptide chains in different contexts, all of them can contribute to cross-linking, making the broad answer the best choice.