The d-electron configurations of $\mathrm{Cr}^{2+}, \mathrm{Mn}^{2+}$, $\mathrm{Fe}^{2+}$ and $\mathrm{Co}^{2+}$ are $d^4, d^5, d^6$ and $d^7$ respectively. Which one of the following will exhibit minimum paramagnetic behaviour? (At. no. $\mathrm{Cr}=24, \mathrm{Mn}=25, \mathrm{Fe}=26, \mathrm{Co}=27$ )

• A

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

  $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}_6\right]^{2+}\right.$
• C

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

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

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