Hint:Let us first understand what is meant by tertiary structure of proteins. The arrangement of the polypeptide chain in space in three dimensions is known as tertiary structure of protein. It contains a single polypeptide chain that has one or more secondary protein structure and is considered as a backbone of tertiary structure.
Proteins exhibit a hierarchical structure. Primary protein structures are formed by combining the amino acids and are held by covalent peptide bonds. They form a linear sequence of amino acids. The secondary protein structures are obtained by the hydrogen bonds that are present between different parts of the linear chain of amino acids.
The tertiary protein structures are obtained by the folding of protein chains. Cysteine has a major contribution in the formation of tertiary protein. Very few amino acids have sulphur in them and cysteine is one of them. The presence of sulphur in cysteine helps it to form bonds in a special way and maintain the protein structure in the body. Two molecules of cysteine are held together by sulfhydryl groups which react together to form a disulphide bridge.
Therefore the correct answer is option (C) i.e, Disulphide bonds.
Note: Proteins are formed by a set of twenty amino acids only. Each of them have a unique side chain. Along with disulphide bonds, the ionic interactions between the positively charged and negatively charged sites on the amino acids and the salt bridges also help in maintaining and stabilizing the tertiary structure of protein.