Conversion of benzene diazonium chloride to chloro benzene is an example of which of the following reactions?

The reaction is known as Sandmeyer reaction. The diazonium compound is treated with cuprous chloride.

$\mathrm{Ar}-{\mathrm{N}}_2^{+}{\mathrm{X}}^{-}\stackrel{\mathrm{CuX}}{⟶}\mathrm{Ar}-\mathrm{X}+{\mathrm{N}}_2$