The relative reactivity of alkyl halides in the SN1 reaction depends on the stability of carbocation. Electron-withdrawing groups decrease the stability and electron-releasing groups increase the stability of carbocations. −NO2 group is an electron-withdrawing group by −M effect. −OMe is electron-releasing groups by +M effect. Hence, the overall order is

∴ Reactivity order (b) > (d) > (c) > (a)
