The difference in the number of unpaired electrons of a metal ion in its high-spin and low-spin octahedral complexes is two. The metal ion is:

• A

  $\mathrm{Ni}^{2+}$
• B

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

  $\mathrm{Co}^{2+}$
• D

  $\mathrm{Mn}^{2+}$

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