Haloarenes (like chlorobenzene) are generally very unreactive towards nucleophilic substitution reactions due to resonance stabilization and the partial double-bond character of the $\text{C-Cl}$ bond. However, their reactivity increases significantly if an electron-withdrawing group (EWG) such as $-\text{NO}_2$ is present at the ortho or para positions . The $-\text{NO}_2$ group at the para position withdraws electron density via both strong $-I$ (inductive) and $-R$ (resonance) effects, making the halogen-bearing carbon more electrophilic and stabilizing the intermediate carbanion formed during the reaction. In contrast, an $-\text{NO}_2$ group at the meta position only exerts a $-I$ effect, which provides less stabilization, and an electron-donating group like $-\text{CH}_3$ decreases reactivity. Therefore, $p$-nitrochlorobenzene undergoes nucleophilic substitution most easily . (Note: Options were reconstructed based on the standard AIPMT 2011 question as they were missing in the raw data).