The aqueous solution of transition metal salt 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 that changes the oxidation state of the metal ion
b. change in 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