Answer
$Ca^{2+}$ falls between the 2 spheres on the left.
$S^{2-}$ falls between the 2 spheres on the right.
Work Step by Step
According to the chart the radii are:
$Mg^{2+}$ 0.86
$K^+$ 1.52
$Cl^-$ 1.67
$Se^{2+}$ 1.84
$Ca^{2+}$ has a radius of 1.14 so falls between $Mg^{2+}$ and $K^+$.
$S^{2-}$ has a radius of 1.70 so falls between $Cl^-$ and $Se^{2+}$.
