To solve this problem, we need to identify the reagent 'R' in the given reaction sequence.$\\$The first step involves converting an aromatic amine to a diazonium salt using $\mathrm{NaNO_2}$ and $\mathrm{HCl}$ at $0-5^\circ \mathrm{C}. \\$The diazonium salt formed is $\mathrm{C_6H_2Br_3N_2^+Cl^-}. \\$The next step involves replacing the diazonium group $(\mathrm{-N_2^+Cl^-})$ with a hydrogen atom.$\\$This is typically achieved using a reduction reaction.$\\$The reagent that can perform this reduction is $\mathrm{CH_3CH_2OH}$ (ethanol), which acts as a reducing agent.$\\$Ethanol reduces the diazonium salt to the corresponding benzene derivative by eliminating $\mathrm{N_2}. \\$Therefore, the reagent 'R' is $\mathrm{CH_3CH_2OH}. \\$This corresponds to Option 2.