The transition of electrons from \(t_{2g}\) to \(e_g\) energy level gives pink colour to the aqueous solution of cobalt (II) chloride \([Co(H_2O)_6]^{2+}\) complex, when excess \(HCl\) is added to the solution it forms \([CoCl_4]^{2-}\), which is blue in colour.
\([Co(H_2O)_6]^{2+}+HCl\rightarrow [CoCl_4]^{2-}\)
pink Excess Blue
Hence, \([Co(H_2O)_6]^{2+}\) is transformed into \([CoCl_4]^{2-}\), it is correct option.
Crystal field splitting energy of tetrahedral complex,
\(\Delta_t=\dfrac{4}{9}\Delta_0\)
Where, \(\Delta_0\) Crystal filed spilitting energy of octahedral complex
\(\because \Delta_t<\Delta_0\)
Tetrahedral complexes have a smaller crystal field splitting than octahedral complexes,
it is correct.
Hence, the correct options are (b) \(\&\) (c).