The correct order of increasing reactivity of CX bond towards nucleophile in the following compounds is

The order of reactivity is dependent on the stability of the intermediate carbocation formed by cleavage of C-X bond. The  $3^0$ carbocation (formed from III) will be more stable than its 2⁰ counter part (formed from IV) which in turn will be more stable than the arenium ion .Also the aryl halide has a double bond character CX bond which makes the cleavage more difficult. However, inspite of all the stated factors, II will be more reactive than I due to the presence of the electron with drawing $-{\mathrm{NO}}_2$ group,C-X bond becomes weak and undergoes nucleophilic substitution reaction