The diamagnetic species is

• Correct answer
  A

  $\left[\mathrm{Ni}(\mathrm{CN})_4\right]^{2-}$
• B

  $\left[\mathrm{NiCl}_4\right]^{2-}$
• C

  $\left[\mathrm{CoCl}_4\right]^{2-}$
• D

  $\left[\mathrm{CoF}_6\right]^{2-}$.

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