The IUPAC name of [ Co( NH3 )6]Cl3 is

Ammonia is a neutral ligand.

So the oxidation state of Co in a complex is

x – 0 = + 3

x = + 3

The IUPAC name is Hexaamminecobalt (III) chloride.

The IUPAC name of the complex is (A) Hexaamminecobalt (III) chloride.