Correct Answer: D
Explanation:
A cofactor is the best general term to describe the non-protein substances required by an enzyme to function properly. This term covers both organic molecules and metal ions. A co-enzyme is an organic molecule that acts as a cofactor. A prosthetic group is a co-enzyme that is covalently bound to the enzyme. A modulator has nothing to do with cofactors and is a ligand that binds to an allosteric binding site.