The function of the enzyme commission (E.C.) number is to:
Identify enzyme reaction specificity is the accurate answer because it correctly identifies the biological function or role described in this question. In Enzymes Basics, understanding the specific functions of molecules, enzymes, or structures is fundamental. Identify enzyme reaction specificity fulfills this particular biological role through its specific structural properties, biochemical activity, or physiological mechanism. The other options (Determine enzyme structure, Classify enzymes based on molecular weight, and Describe only protein-based enzymes) serve different biological functions or are associated with other processes, pathways, or structural roles within the cell or organism.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 6