The correct option is B Ester bonds
The secondary structure folds upon itself to form the 3 dimensional shape of the protein. This is the "tertiary structure" or the sub-unit of protein.
Hydrophobic interactions, H-bonds and electrostatic interactions are some of the weak interactions that stabilize the tertiary structure of proteins.
Ester bonds are covalent bonds. Phosphodiester bonds are a type of ester bond that join many nucelotides together to form a nucleotide chain. Lipids have fatty acid chains that are linked to the glycerol backbone through ester bonds.
However, ester bonds are least likely to be involved in maintaining the tertiary structure of proteins.