In the questions, arrange the compounds in increasing order of the rate of reaction towards nucleophilic substitution.
(i) (a) < (b) < (c)
(ii) (a) < (c) < (b)
(iii) (c) < (b) < (a)
(iv) (b) < (c) < (a)
The order of the rate of reaction towards nucleophilic substitution depends on the presence of electron-withdrawing groups and the number of electron-withdrawing groups. The presence of electron-withdrawing groups at the ortho and para positions increases the rate of nucleophilic substitution. The more electron-withdrawing groups there are, the higher the reactivity. The rate of nucleophilic substitution decreases when there is an electron releasing group at ortho or para positions.
Hence, the answer is the option (iv).