Concept:
Nucleophilic substitution reactions are easier when the carbon-halogen bond can break easily.
In alkyl halides and benzyl halides, the carbon-halogen bond is relatively easier to break.
But in aryl halides, the halogen is directly attached to the benzene ring.
In aryl halides, the \(C-Cl\) bond gets partial double bond character due to resonance.
Step 1: Identify the type of halide in each option.
Option (A), \(C_6H_5CH_2Cl\), is benzyl chloride.
It is highly reactive towards nucleophilic substitution because the benzyl carbocation is resonance stabilized.
\[
C_6H_5CH_2Cl \Rightarrow \text{benzyl halide}
\]
Option (C), \(CH_3Cl\), is methyl chloride.
It can undergo \(S_N2\) reaction.
Option (D), chlorocyclohexane, is an alkyl halide.
Option (B), \(p\)-chlorotoluene, is an aryl halide.
\[
p\text{-chlorotoluene} \Rightarrow \text{aryl chloride}
\]
Step 2: Explain why aryl chloride is least reactive.
In aryl chloride, chlorine is directly attached to the benzene ring.
The lone pair on chlorine participates in resonance with the benzene ring.
Because of this, the \(C-Cl\) bond develops partial double bond character.
\[
C-Cl \Rightarrow \text{partial double bond character}
\]
This makes the bond shorter and stronger.
Therefore, breaking this bond becomes difficult.
Step 3: Final conclusion.
Since \(p\)-chlorotoluene is an aryl halide, it is least reactive towards nucleophilic substitution.
Hence:
\[
\boxed{\text{(B) }p\text{-Chlorotoluene}}
\]
