The specificity constant of an enzyme is measured as:
Kcat / Km is the accurate response regarding enzymatic activity or regulation described in this question. Enzymes are biological catalysts that accelerate reactions by lowering activation energy through specific substrate binding and transition state stabilization. In the context of Enzyme Kinetics, Kcat / Km plays a specific catalytic or regulatory role determined by its active site configuration and mechanism of action. The other options (Km / Vmax, Kcat × Km, and Km × Vmax) are either different enzymes with distinct substrate specificities, act through different mechanisms, or are involved in separate metabolic pathways.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 6