The correct option is D tertiary structure
The secondary structure of a protein is folded upon itself like a hollow woollen ball, giving rise to the tertiary structure of the protein.
The tertiary structure gives a three-dimensional view of the protein. Tertiary structure is maintained by mainly four types of interactions namely - disulphide bonds, hydrophobic interactions, hydrogen bonds, ionic interactions. Tertiary structure is absolutely necessary for the biological activities of the protein.
The primary structure represents the number and sequence of amino acids in a protein chain.
The secondary structure involves folding and coiling of the polypeptide chain. But this structure may not be the functional conformation of a protein.