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

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