The green coloured complex ion of nickel in its aqueous solution is 

Complexes containing metal ions in their lowest oxidation states may be colourless or light yellow. 

In such cases, the ligands are all of the same types and have a very low ionization potential. 

They are thus able to supply electrons quickly so that almost all of the electrons from the reducible metal ion are localized as singlets on the metal ion. This process is called electron transfer from ligand to the metal ion, and under these conditions, it is essentially complete by 0°C. 

Thus, these complexes are colourless or nearly colourless at room temperature and show essentially no colour changes with temperature up to a certain maximum temperature. 

Hence, option (D) is correct answer.