Stability of the toxoid in snake's venom is due to:

Ans is 'a' i.e. Disulfide bonds:"A disulfide linkage contribute to the stability of the three dimensional shape of the protein molecule. For example, many disulphide bonds are found in proteins that are secreted by cells. It is thought that these strong covalent bonds help stabilise the structure of proteins and prevent them from becoming denatured in the extracellular environmentSince snake's venom is a protein secretion it is most probably stabilized by disulfide bonds.