The existence of two different coloured complexes with the composition of ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{+}$ is due to

• A

  linkage isomerism

• B

  geometrical isomerism

• C

  coordination isomerism

• D

  ionisation isomerism