Which ion is not isoelectronic with $\mathrm{Ar}$?

${\mathrm{Sc}}^{2+}$ ion is not isoelectronic with $\mathrm{Ar}$ which is equals to 18 electrons.

Atoms and ions with the same amount of electrons are said to be isoelectronic species. Or, an isoelectronic pair is a pair of species with the same amount of electrons that combines all the atoms of a certain species. The radii of isoelectronic species would vary due to varying nuclear charges.

Hence, the option C is correct.