The correct answer is: A. II > I > III
The reactivity of a compound towards electrophiles depends on the electron density in the compound. The more electron-rich the compound, the more reactive it will be towards electrophiles.
In the given compounds, the electron density is highest in compound II, followed by compound I, and lowest in compound III. This is because compound II has a phenyl group, which is an electron-donating group. The phenyl group donates electrons to the ring, making the ring more electron-rich. Compound I also has a phenyl group, but it is not as electron-donating as the phenyl group in compound II. This is because the phenyl group in compound I is attached to a carbonyl group, which is an electron-withdrawing group. The carbonyl group withdraws electrons from the ring, making the ring less electron-rich. Compound III does not have any electron-donating groups, and it has an electron-withdrawing group (the carbonyl group). This makes compound III the least electron-rich of the three compounds.
Therefore, the decreasing order of the reactivity of the following compounds towards electrophiles is:
II > I > III