Final answer:
The physical meaning of Km (Michaelis constant) is the enzyme-substrate binding affinity (option a).
Step-by-step explanation:
The Km is the substrate concentration at which the velocity of the enzyme-catalyzed reaction is half the maximum velocity. It represents the affinity between the enzyme and the substrate, indicating how tightly the enzyme can bind to the substrate. A higher Km value indicates weak binding, while a lower Km value indicates strong binding.
Hence, The correct answer is a) Enzyme-substrate binding affinity.