When aqueous sodium hydroxide is added to an aqueous solution of chromium (III) ions, a green blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to :
a. [Cr(H2O)6]3+
b. CrO42−
c. [Cr(OH)4]−
d. [Cr(OH)3(H2O)3]