The correct option is B Presence of an enzyme inhibitor in the reaction mixture
The graph given shows the relationship of rate or velocity of the reaction with respect to substrate concentration.
Option( a) is true only till the point where velocity reaches the maximum. After that, as the substrate concentration is increased, it is seen in the graph that the velocity decreases. As this decrease in the velocity of the reaction upon further addition of substrate is not explained by option (a), hence it is incorrect.
After the reaction reaches Vmax, further addition of substrate causes a decline in the rate of reaction, indicating the presence of inhibitor molecules in the reaction mixture. The reaction mixture includes the enzyme, substrate, product and the inhibitor in this case. Hence, option (b) is correct.
Formation of the enzyme-substrate complex may or may not take place after the addition of inhibitors as the inhibitor may be competitive or non-competitive. The enzyme-substrate complex is formed only till the velocity of the reaction reaches its peak. Beyond that it is decreasing in the graph and this is not explained in option (c). Hence, the option (c) is incorrect.
The given graph is the velocity of reaction vs substrate concentration graph. The relation between pH and substrate concentration for this particular reaction cannot be deduced by this graph.