Right choice is (a) The effect of diffusional control on the local pH
The best I can explain: The graph in the diagram represents the effect of diffusional control on the local pH. This graph shows the variation of surface pH and bulk pH at various proton moduli (µH). The blue line represents the variation at µH = 10^-5 and red dotted line represents the variation at different proton moduli (µH = 10, µH = 10^-1v, µH = 10^-3, µH = 10^-4). At high pH, the micro-environmental pH reduced due to presence of very low hydrogen ion concentration gradient and hydrogen ion diffusion rates. From the plot, it is found that hydrogen ions accumulate at the surface when the proton modulus is greater than about 10,000. Hence the reactions must be operated at a significant rate away from the pH optimum of the free enzyme.