Final answer:
Disulfide bonds are responsible for holding two insulin subunits together after the pro insulin portion is removed, stabilizing the insulin's quaternary structure.
Step-by-step explanation:
The interaction that keeps two insulin subunits together after the pro portion leaves is d) Disulfide bonds. These are covalent bonds that form between the sulfur atoms of cysteine residues within a polypeptide chain or between two different chains. Specifically, in insulin, the disulfide linkages hold the A and B chains together after the pro portion has been cleaved off, stabilizing the quaternary structure of the protein. Disulfide bonds are much stronger than hydrogen bonds and hydrophobic interactions, and unlike ionic bonds, are covalent, making them crucial for the stability of proteins that experience changes in the environment such as insulin.