D) a protein segment that folds independently
Explanation:
The tertiary structure refers to unique three- dimensional conformation that globular proteins assume as a result of interaction between their primary structure.
The interactions that stabilize the tertiary structures are, Hydrophobic interaction, Electrostatic interaction, Hydrogen bonds, Van der Waals force of interaction and covalent bonds.
The fundamental structure of a tertiary structure is the Domain.
It is a polypeptide chain or a part of polypeptide chain that can fold into stable three dimensional-tertiary structure.
