The conversion of benzene diazonium chloride to bromobenzene can be accomplished by

The diazonium group $$\left(-N_2^+\;Cl^-\right)$$ in benzene diazonium chloride can be replaced by a halogen atom through two closely related name reactions.
  • Sandmeyer reaction: uses a cuprous halide, $$CuX$$, in the presence of the corresponding halogen acid to introduce $$X = Cl, Br$$ or $$CN$$.
  • Gattermann reaction: uses finely divided copper metal and the corresponding hydrogen halide, HX, to introduce $$X = Cl$$ or $$Br$$.

To obtain bromobenzene from benzene diazonium chloride we need to introduce a bromine atom. This is achieved by treating the diazonium salt with copper powder and hydrobromic acid:

$$C_6H_5N_2^+Cl^- \;+\; Cu \;+\; HBr \;\longrightarrow\; C_6H_5Br \;+\; N_2 \;\uparrow\;+\; CuCl$$

This copper + HX route is specifically called the Gattermann reaction. Hence the correct choice is:

Option C which is: Gattermann reaction