To determine which reaction proceeds through electrophilic substitution, let's analyze each option:$\\$Option 1:$\\$The reaction involves the conversion of a diazonium salt to a chlorobenzene using $Cu_2Cl_2. \\$This is a Sandmeyer reaction, which proceeds through a radical mechanism, not electrophilic substitution.$\\$Option 2:$\\$The reaction is the chlorination of benzene using $Cl_2$ in the presence of $AlCl_3. \\$This is a classic example of electrophilic aromatic substitution (EAS), where the $Cl^+$ acts as the electrophile.$\\$Option 3:$\\$The reaction involves the addition of chlorine to benzene under UV light.$\\$This is a free radical substitution, not electrophilic substitution.$\\$Option 4:$\\$The reaction involves the conversion of benzyl alcohol to benzyl chloride using HCl.$\\$This is a nucleophilic substitution reaction, not electrophilic substitution.$\\$Therefore, the correct option is Option 2, where the reaction proceeds through electrophilic substitution.$\\$Correct Option: 2