The reactivity of an aromatic ring towards an electrophile depends on the electron density of the ring. Electron-donating groups increase the electron density, thereby increasing reactivity, while electron-withdrawing groups decrease the electron density, thereby decreasing reactivity. In part (a), the nitro group is a strong electron-withdrawing group. Chlorobenzene has one weakly deactivating chlorine atom, p-nitrochlorobenzene has one chlorine and one nitro group, and 2,4-dinitrochlorobenzene has one chlorine and two nitro groups. Thus, the deactivating effect increases with the number of nitro groups. In part (b), the methyl group is electron-donating, making toluene the most reactive. The p-O₂N–C₆H₄–NO₂ compound has two strongly deactivating nitro groups, making it the least reactive.