The aqueous solution of salt of a transition metal ion changes colour from pink to blue, when concentrated hydrochloric acid is added to it. The change in colour is due to:
A. Evolution of hydrogen the changes the oxidation state of the metal ion
B. Change ini the coordination number of the metal ion from 6 to 4 and formation of new species in solution
C. formation of a coordination complex of the metal ion with hydrochloric acid
D. protonation of the metal ion.