In the question arrange the compounds in increasing order of the rate of reaction towards nucleophilic substitution.
(i) (a) < (b) < (c)
(ii) (c) < (b) < (a)
(iii) (a) < (c) < (b)
(iv) (c) < (a) < (b)
(iii) is an electron-withdrawing group and when it is at ortho and para position it causes nucleophilic substitution. Where the effect of this same electron-withdrawing group is very less when it is at the meta position.