The increasing order of the ionic radii of the given isoelectronic species is:

For isoelectronic species (all have 18 electrons here), the ionic radius decreases as the nuclear charge (atomic number $Z$) increases.Species: ${\text{S}}^{2-}(Z=16)$, ${\text{Cl}}^{-}(Z=17)$, ${\text{K}}^{+}(Z=19)$, ${\text{Ca}}^{2+}(Z=20)$.Order of $Z$: $\text{S}<\text{Cl}<\text{K}<\text{Ca}$.Order of Radius: ${\text{Ca}}^{2+}<{\text{K}}^{+}<{\text{Cl}}^{-}<{\text{S}}^{2-}$.