The reaction $A r \mathrm{~N}_{2}^{+} \mathrm{Cl}^{-} \stackrel{\mathrm{Cu} / \mathrm{HCl}}{\longrightarrow} \mathrm{ArCl}+\mathrm{N}_{2}+\mathrm{CuCl}$ is called as

The reaction of an arenediazonium salt with copper powder in the presence of the corresponding halogen acid (HCl or HBr) to replace the diazonium group with a halogen atom is known as the Gattermann reaction.

While the Sandmeyer reaction uses copper(I) salts (CuCl or CuBr) as catalysts, the Gattermann reaction specifically utilizes finely divided copper powder with the corresponding acid. Therefore, the given reaction is the Gattermann reaction.

Answer: Gattermann reaction