When concentrated $HCl$ is added to an aqueous solution of $CaCl_2$, its $coCl_2$ its colour changes from reddish pink to deep blue. Which complex ion gives blue in this reaction ?

Question

$\begin{array}{1 1} [CoCl_6]^{4-} \\ [CoCl_6]^{3-} \\ [CoCl_4]^{2-} \\ [Co(H_2O)_6]^{2+} \end{array}$