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