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

Pink colour is due to transition of electrons from t_2g to e_g orbitals. When an excess of HCl is added, [Co(H₂O)₆]²⁺ is transformed into [CoCl₄]^2–. Also, tetrahedral complexes have smaller crystal field splitting than octahedral complexes, so the crystal field splitting energy is low, the wavelength absorbed will be higher(towards red) but the complementary colour observed will be blue.

Thus, Options B & C are the correct answers.