${\text{Cu}}^{2+}\left(\text{aq}\right)+{\text{X}}^{-}\left(\text{aq}\right)\stackrel{\text{R}.\text{T}.}{\to }\text{CuX}\downarrow +{\text{X}}_2$ , 'X' cannot be

• Correct answer
  A

  $\mathrm{C}{\mathrm{l}}^{-}\left(\mathrm{a}\mathrm{q}\right)$
• B

  ${\mathrm{I}}^{-}\left(\mathrm{a}\mathrm{q}\right)$
• C

  $\mathrm{C}{\mathrm{N}}^{-}\left(\mathrm{a}\mathrm{q}\right)$
• D

  $\mathrm{S}\mathrm{C}{\mathrm{N}}^{-}\left(\mathrm{a}\mathrm{q}\right)$

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