The coordination number of Co in the complex compound $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_3$ is 

The coordination number of Co in the complex compound $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_3$ is 6.

The coordination number of the metal atom or ion refers to the number of ligand atoms that are directly attached to the core metal atom or ion by the coordinate bond. Six ammonia molecules surround Co, which is chemically linked to the metal. Consequently, the given compound's coordination number will be 6.

Hence, option D is correct.