Select the correct IUPAC name for: $[Co(NH_3)_6]^{2+}$

Question

$\begin{array}{1 1}(a)\;\text{hexammoniacobaltate(II) ion}\\(b)\;\text{hexaamminecobaltate(II) ion}\\(c)\;\text{hexammoniacobalt(II) ion}\\(d)\;\text{hexaamminecobalt(II) ion}\end{array}$