Which one of the following transition metal ions is diamagnetic?

• A

  ${\mathrm{C}\mathrm{o}}^{2+}$
• B

  ${\mathrm{N}\mathrm{i}}^{2+}$
• C

  ${\mathrm{C}\mathrm{u}}^{2+}$
• Correct answer
  D

  ${\mathrm{Z}\mathrm{n}}^{2+}$

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