An aqueous pink solution of cobalt(II) chloride changes to deep blue on addition of excess of HCI. This is because..............

Cobalt (II) chloride's aqueous pink solution is caused by an electron moving from the 

${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2-}$ complex's ${\mathrm{t}}_{2_{\mathrm{g}}}$ to eg energy state. If too much HCl is added, this solution.

(i) ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2-}$ is transformed into  ${\left[{\mathrm{CoCl}}_4\right]}^{2-}$.

(ii) Tetrahedral complexes have smaller crystal field splitting than octahedral complexes because 

$\mathrm{\Delta }r=\frac{4}{9}{\mathrm{\Delta }}_0$

Hence, options B and C are correct.