Step 1: Understanding the Question:
Aryl halides are generally unreactive towards nucleophilic substitution due to partial double bond character of the $C-X$ bond. This reactivity can be increased by the presence of electron-withdrawing groups (EWGs).
Step 2: Detailed Explanation:
- The presence of an EWG like the nitro group ($-NO_{2}$) at ortho or para positions stabilizes the negative charge in the intermediate Meisenheimer complex through resonance (mesomeric effect).
- This makes the $C-Cl$ bond easier to break (increases reactivity).
- In m-nitrochlorobenzene, the nitro group cannot stabilize the carbanion intermediate via resonance because the negative charge never resides on the carbon atom bearing the nitro group.
- Therefore, the meta isomer is less reactive than the ortho and para isomers.
- 2,4,6-trinitrochlorobenzene has three such groups and is the most reactive.
- Consequently, m-nitrochlorobenzene has the most difficulty in breaking the $C-X$ bond among the given substituted chlorobenzenes.
Step 3: Final Answer:
m-Nitrochlorobenzene has the most difficulty in breaking the $C-X$ bond compared to the other options.