Four species have 18 electrons each: P has Z = 20, Q has Z = 17, R has Z = 19 and S has Z = 16. Their sizes in increasing order are

For isoelectronic species, a larger nuclear charge pulls the electrons in more strongly and gives a smaller radius. So the radius order is Z = 20 < 19 < 17 < 16, i.e. $\mathrm{Ca}{}^{2+}<\mathrm{K}{}^{+}<\mathrm{Cl}{}^{-}<\mathrm{S}{}^{2-}$, which is P < R < Q < S.