All of these are correct. The tertiary structure of a protein is stabilized by various types of interactions, including:
- Hydrogen bonding: Weak attractions between polar side chains or between polar side chains and the backbone.
- Van der Waals forces: Weak attractions that occur between nonpolar side chains when they are in close proximity.
- Ionic bonding: Electrostatic attractions between positively and negatively charged side chains.
- Covalent bonding: Disulfide bridges can form between cysteine residues, providing strong covalent links that stabilize the structure.
Therefore, all of these forces and bonding types contribute to a protein's tertiary structure.