To solve this problem, we need to determine which compound undergoes an $S_N1$ reaction at the fastest rate.$\newline$The $S_N1$ reaction rate depends on the stability of the carbocation formed after the leaving group departs.$\newline$Let's analyze each option:$\newline$Option 1:$\newline$• The compound is bromobenzene.$\newline$• The leaving group is directly attached to the benzene ring.$\newline$• Aryl halides do not form stable carbocations, so $S_N1$ is unlikely.$\newline$Option 2:$\newline$• The compound is 1-phenylethyl bromide.$\newline$• The leaving group is attached to a secondary carbon adjacent to a phenyl group.$\newline$• The resulting carbocation is benzylic, which is highly stabilized by resonance with the phenyl ring.$\newline$Option 3:$\newline$• The compound is cyclohexylmethyl bromide.$\newline$• The leaving group is attached to a primary carbon.$\newline$• Primary carbocations are not stable, so $S_N1$ is less favorable.$\newline$Option 4:$\newline$• The compound is cyclohexyl bromide.$\newline$• The leaving group is attached to a secondary carbon.$\newline$• The resulting carbocation is secondary, which is more stable than primary but less stable than benzylic.$\newline$Conclusion:$\newline$• Option 2 forms a benzylic carbocation, which is the most stable among the options.$\newline$• Therefore, the compound in Option 2 will undergo the $S_N1$ reaction at the fastest rate.$\newline$This corresponds to Option 2.