Which of the following compounds undergoes nucleophilic substitution reaction most easily?

In S_N Ar reactions, acarbanion is formed as an intermediate, so any substituent that increases the stability of carbanion and hence the transition state leading to its formation will enhance the S_NAr reactions. To compare the rates of substitution in chlorobenzene, chlorobenzene having electron-withdrawing group, and chlorobenzene having electron-releasing group, we compare the structures carbanion I (from chlorobenzene), II (from chlorobenzene containing electron-withdrawing group) and III (from chlorobenzene containing electron-releasing group). 

G withdraws electrons, neutralises (disperses) —ve charge of the ring, stabilises carbanion, facilitates S_N, reaction (activation effect) 

G releases electrons, intensifies —ve charge, destabilizes carbanion, retards S_N reaction (deactivation) NO₂ is activating group and CH₃, and OCH₃, are deactiving group.Hence, the correct order of nucleophilic substitution reactions