Which of the following complexes is expected to be colored?

• A

  $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$
• B

  $\left[\mathrm{Zn}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$
• Correct answer
  C

  $\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$
• D

  $\left[\mathrm{Mg}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$

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