The tertiary structure of a polypeptide is the select one:
a. localized region of a polypeptide chain that forms an a helix or b pleated sheet.
b. overall three-dimensional shape of a protein composed of more than one polypeptide.
c. overall three-dimensional shape of a fully folded polypeptide.
d. linear sequence of amino acids in a polypeptide.

The correct answer is C. The tertiary structure of a polypeptide is the overall three-dimensional shape of a fully folded polypeptide.

Explanation:

A polypeptide is a molecular chain composed of at least 10 amino acids (which are the molecules that make up proteins). When we talk about its tertiary structure, it refers to the complete overall three-dimensional structure of the polypeptide units of a given protein, where the polypeptide chain is fully folded and compacted. This folding is facilitated by unions called disulfide bonds, which are created from the cysteine residues, these bonds (called disulfide bridges as well) help to  stabilize many polypeptides.