The reaction sequence is: 1. Sandmeyer reaction of benzenediazonium chloride ($C_6H_5N_2Cl$) with $Cu_2Br_2/HBr$ gives bromobenzene ($C_6H_5Br$). 2. Reaction of bromobenzene with Mg in dry ether forms the Grignard reagent, phenylmagnesium bromide ($C_6H_5MgBr$). 3. Hydrolysis of the Grignard reagent with $H_2O$ yields benzene ($C_6H_6$).