The reactivity of aryl halides to nucleophilic substitution is extremely low due to the resonance stabilization of the benzene ring. The –Cl bond also acquires a partial double bond due to resonance. Presence of an electron withdrawing –NO₂ group at ortho or para positions on the ring increases the reactivity of aryl halides. The presence of –NO₂ at a closer position to the C—Cl makes the molecule more reactive. The order of reactivity from highest to lowest should be (b) > (c) > (a). The correct option is (iii).