Correct choice is (a) Effect of the polarity of A and B

To explain I would say: The plot represents the effect of polarity he polarity of A and B, keeping the ratio of the partition coefficients constant (PB/PA = 100). This plot is drawn considering the variation in the apparent equilibrium constants of a one-substrate one-product reaction with the relative component composition of a biphasic aqueous-organic system. The following values for the partition coefficients have been used, from the top downwards; PB = 100, PB = 10, PB = 1, PB = 0.1, PB = 0.01, PB = 0.001, PB = 0.0001.