In the complex $\left[\mathrm{Pt}\left(\mathrm{NH}_3\right)_2 \mathrm{Cl}_2\right]$, platinum is in the +2 oxidation state. The ammonia $\left(\mathrm{NH}_3\right)$ ligands are named as "ammine" and the chloride ($\mathrm{Cl}$) ligands are named as "chloro". According to IUPAC naming rules, the ligands are listed alphabetically before the central metal ion, and the oxidation state of the metal is indicated in Roman numerals. Therefore, the correct name is Dichloro diammine platinum (II).