I) ${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2\mathrm{Cl}\stackrel{\mathrm{CuCl}/\mathrm{HCl}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{N}}_2$

II) ${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2\mathrm{Cl}\stackrel{\mathrm{CuC}/\mathrm{HCl}}{\to }{C}_6{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{N}}_2+\mathrm{CuCl}$

Correct statement among the following is

Sandmeyers reaction is the substitution reaction in which benzene diazonium chloride reacts with $\mathrm{CuCl}/\mathrm{HCl}$ to form chloro benzene along with ${\mathrm{N}}_2$.

And the reaction of benzene diazonium chloride with $\mathrm{CuCl}/\mathrm{HCl}$ to form chlorobenzene along with ${\mathrm{N}}_2$ and $\mathrm{CuCl}$ as the byproduct is known as the Gattermann reaction.

And the yield of chlorobenzene is more in reaction I than in reaction II as, in reaction II there are more byproducts in it.

Hence, all the statements are correct.

Therefore, option (D) is correct.