Final answer:
Zinc (Zn2+) functions as a cofactor for enzyme B in the catalysis of substrate X. Cofactors are essential non-protein components that enable enzymatic reactions, and zinc, being an inorganic metal ion, fits this category, distinguishing it from coenzymes, substrates, and products.
Step-by-step explanation:
In the context of enzyme activity, zinc (Zn2+) is best identified as a cofactor. A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's biological activity to occur. These cofactors may either be bound permanently to enzymes or may bind loosely and reversibly along with the substrate during catalysis.
In the case of enzyme B catalyzing the conversion of substrate X with Zn2+, zinc does not function as a coenzyme because coenzymes are organic helper molecules. Nor is zinc a substrate (a substance on which an enzyme acts) or a product (a substance that results from an enzymatic reaction). Therefore, zinc, which is an inorganic metal ion, acts as a cofactor that is essential for the enzyme's activity.