Which one of the following is a diamagnetic ion?

Sc³⁺

The answer is (c) \(Sc^{3+}\).

A diamagnetic ion is an ion that has no unpaired electrons. This means that the magnetic moment of the ion is zero.

The ions $Cu^{2+}, Mn^{2+}, \text{ and } Co^{2+}$ all have unpaired electrons, so they are paramagnetic. The ion \(Sc^{3+}\), on the other hand, has a completely filled d-orbital, so it is diamagnetic.

Here is a table of the magnetic properties of the ions: