Final answer:
In the absence of water, ionic bonds are strong between amino acids due to the electrostatic attraction between oppositely charged side chains, contributing significantly to the maintenance of a protein's structure. Therefore, correct option is c.
Step-by-step explanation:
When water is not present, ionic bonds are strong between amino acids. Ionic bonds are a type of electrostatic interaction that occurs between positively and negatively charged side chains of amino acids.
For example, they help maintain the folded structure of proteins by creating attractions like those between an aspartic acid carboxylate ion and a lysine ammonium ion.
Other types of bonds include hydrogen bonds, which are weaker and involve a hydrogen atom interacting with an electronegative oxygen or nitrogen atom, and disulfide bonds, which are covalent and occur between the sulfur atoms of two cysteine amino acids, considerably stabilizing the protein's tertiary structure.
Therefore, the correct answer to the student's question is C) Ionic.