Final answer:
Cysteine amino acids in proteins can form disulfide bonds, which are covalent bonds essential for stabilizing the protein's tertiary structure. This structure is important for the protein's overall shape and function. The correct option is C).
Step-by-step explanation:
Among the options given, the molecule that has a functional group that frequently forms covalent bonds to maintain the tertiary structure of a protein is cysteine.
In proteins that contain cysteine amino acids, the sulfur atoms can form disulfide bonds. These bonds are covalent and help in stabilizing the protein's tertiary structure, which then can contribute to the overall three-dimensional shape essential for the protein's function.
The tertiary structure is supported by various kinds of bonds and interactions, but the unique disulfide bond is the main covalent bond that stabilizes this level of protein organization. Additionally, proteins may bind metal ions or small organic molecules, and feature domains and motifs relevant to their specific functions. Option C) is the correct one.