Reaction Mechanism

Nucleophile attacks readily to the compound that has least electron density. Electron withdrawing groups like etc decrease the electron density while electron releasing groups like increase the electron density at benzene nucleus.
Compound (iv) due to the presence of four electron withdrawing groups has the least electron density and thus, is more reactive towards nucleophilic substitution.
Hence, the correct order of reactivity towards nucleophilic substitution is as

The case with which a nucleophile attacks the carbonyl group depends upon the electron deficiency, i.e. the magnitude of positive charge on carbonyl group carbon. Since, among all the groups attached

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has maximum electron withdrawing effect, electron deficiency will be maximum at carbonyl carbon of acetyl chloride. Hence, acetyl chloride, i.e.


is most reactive to nucleophilic attack at carbonyl group