Solution

p-nitrochlorobenzene undergoes nucleophilic substitution reaction most easily and p-methoxy chlorobenzene undergoes nucleophilic substitution least easily.

When strong electron-withdrawing nitro group is present, the aromatic ring becomes more electrophilic and the nucleophile can easily attack.

When electron releasing group such as methyl or methoxy group is present, the aromatic ring becomes less electrophilic which decreases the ease with which the nucleophile can attack.

The decreasing order of the ease of the nucleophilic substitution reaction is as shown below.

Hence, the correct option is A.