The oxidation number of the underlined atom in the following species

(Identify the incorrect statement)

• Correct answer
  A

  ${\mathrm{Cu}}_2\underset{¯}{\mathrm{O}}$ is -1
• B

  $\underset{¯}{\mathrm{Cl}}{\mathrm{O}}_3^{-}$ is +5
• C

  $\mathrm{Kr}\underset{¯}{{\mathrm{Cr}}_2}{\mathrm{O}}_7$ is $+6$
• D

  $\mathrm{H}\underset{¯}{\mathrm{Au}}{\mathrm{Cl}}_4$ is $+3$

Hence, the correct answer is option: 'A'.