Reagent used in Etard reaction is

• A

  ${\mathrm{CrO}}_2{\mathrm{Cl}}_2 + {\mathrm{CS}}_2 / {\mathrm{H}}_3{\mathrm{O}}^{+}$

• B

  ${\mathrm{CrO}}_3 + {\left({\mathrm{CH}}_3\mathrm{CO}\right)}_2 / {\mathrm{H}}_3\mathrm{O}+$

• C

  $\mathrm{CO} + \mathrm{HCl} / \mathrm{CuCl}$

• D

  ${\mathrm{Cl}}_2 / 373 \mathrm{K} / {\mathrm{H}}_3{\mathrm{O}}^{+}$