Which one of the following ions exhibits the highest magnetic moment?

Mn²⁺

The correct answer is option 4. \(Mn^{2+}\).

The magnetic moment of an ion is a measure of the strength of its magnetic field. It is determined by the number of unpaired electrons in the ion's d-orbital.

The ions $Cu^{2+}, Ti^{3+},\text{ and } Ni^{2+}$ all have unpaired electrons in their d-orbitals, but $Mn^{2+}$ has the highest number of unpaired electrons (5). Therefore, $Mn^{2+}$ has the highest magnetic moment.

Here is a table of the magnetic moments of the ions in Bohr magnetons :

As you can see, \(Mn^{2+}\) has the highest magnetic moment of the four ions.