Answer:-    I < II < III < IV

The reactivity order is I < II < III < IV. This trend can be attributed to the differing reactivity of alkyl halides compared to aryl halides. Alkyl halides exhibit higher reactivity due to the presence of partial double bond character between the benzene ring and the halogen atom. Additionally, in alkyl halides, the reactivity increases with greater carbocation stability. Consequently, tertiary halides are more reactive. Meanwhile, the presence of a nitro group, which acts as an electron-withdrawing group, enhances the reactivity of aryl halides.