In the complex $[\mathrm{Co(NH_3)_5\,Cl}]\mathrm{Cl_2}$, the oxidation state of cobalt is

1. Total charge on the complex $[\mathrm{Co(NH_3)_5\,Cl}]\mathrm{Cl_2}$ is zero (neutral compound). 2. The OUTER chloride ions ($\mathrm{Cl_2}$) have total charge $-2$. So the complex ION $[\mathrm{Co(NH_3)_5\,Cl}]$ must carry charge $+2$. 3. Inside the complex ion, neutral ligands NH$_3$ contribute 0; the inner Cl$^-$ contributes $-1$. 4. Let oxidation state of Co be $x$: $x + 5(0) + (-1) = +2 \Rightarrow x = +3$. 5. So Co is in the $+3$ oxidation state. This is consistent with cobalt's preferred octahedral $\mathrm{Co^{3+}}$ chemistry. 6. Option A would require the complex ion to be $0$; B would require $+1$; D would require $+3$. _Source: syllabus Chemistry Part 1, Ch 5, §5.3 (Oxidation number from neutral compound charge), p. 3–4._