Which one of the following ions exhibits colour in aqueous solution?

Ni²⁺

The correct answer is option 3. \(Ni^{2+}\).

The ions $Ti^{4+}, Zn^{2+}, \text{ and } Sc^{3+}$ all have a complete d-orbital, so they are colorless. The ion \(Ni^{2+}\) has an incomplete d-orbital, so it can absorb light and exhibit color.

The color of an ion is due to the d-d transitions of the electrons in the d-orbital. When light is absorbed, it causes an electron to move from one energy level to another. The difference in energy between the two energy levels is equal to the energy of the light that is absorbed.

In the case of \(Ni^{2+}\), the d-orbital is not completely filled, so there are different energy levels available for the electrons to move between. This allows \(Ni^{2+}\) to absorb a range of wavelengths of light, which gives it a characteristic color.

The other ions do not have any unpaired electrons in the d-orbital, so they cannot absorb light and exhibit color.