Q.Which of the following compounds undergo nucleophilic substitution reaction most easily?

A strong electron-withdrawing group positioned ortho/para to the leaving halogen best stabilises the negatively-charged intermediate of nucleophilic aromatic substitution, and para-nitro gives the best such activation among the options.

Unlike simple alkyl halides, aryl halides normally resist nucleophilic substitution. However, this resistance is overcome when a strong electron-withdrawing group (such as $-NO_2$) is present at the ortho or para position relative to the halogen — such a group stabilises the negatively-charged Meisenheimer (addition) intermediate by delocalising the extra electron density onto itself, making the substitution proceed via an addition-elimination mechanism.

Comparing the options:

• (a) 1-chloro-4-nitrobenzene: $-NO_2$ is para to $-Cl$ — directly conjugated, strongly stabilising → most reactive.
• (b) 1-chloro-3-nitrobenzene: $-NO_2$ is meta to $-Cl$ — not conjugated with the leaving group's position, so it cannot stabilise the intermediate by resonance → much less activating. …