In the question arrange the compounds in increasing order of rate of reaction towards nucleophilic substitution.
(a) (iii) < (ii) < (i) (b) (ii) < (iii) < (i)
(c) (i) < (iii) < (ii) (d) (i) < (ii) < (iii)
Presence of electron withdrawing group at ortho and para position facilitate the nucleophilic substitution reaction and hence, enhances the rate of reaction.
Compound (iii) has three electron withdrawing groups at ortho and para positions w.r.t. chlorine while compound (ii) has only one electron withdrawing group and there is no electron withdrawing group in compound (i).
So, the increasing order of rate of reaction towards nucleophilic substitution is d). (i) < (ii) < (iii).