Co-ordination number of platinum ${\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{2+}$ ion is:

Co-ordination number of tetraamminedichloroplatinum(iv) tetrachloroplatinate(ii)${\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{2+}$ is 2.

Co-ordination number = denticity of the central atom X number of ligands attached to it

Co-ordination number = (4+2)x 1

Co-ordination number = 6

Since both of the ions in this situation, 4 ammonia and 2 chloride, have denticities of 1, the co-ordination number would be 6.

Hence, correct option is D.