Type of isomerism exhibited by compounds $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right] \mathrm{Cl}_3,\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{Cl}^{\mathrm{Cl}} \mathrm{Cl}_2 \cdot \mathrm{H}_2 \mathrm{O},\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_4 \mathrm{Cl}_2\right] \mathrm{Cl}_2 . \mathrm{H}_2 \mathrm{O}\right.$ and the value of coordination number $(\mathrm{CN})$ of central metal ion in all these compounds, respectively is:

• A

  Ionisation isomerism, $\mathrm{CN}=4$
• Correct answer
  B

  Solvate isomerism, $\mathrm{CN}=6$
• C

  Geometrical isomerism, $\mathrm{CN}=4$
• D

  Optical isomerism, $\mathrm{CN}=4$

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