When the atoms: Ba, Cs, Mg, Na are arranged in order of increasing size, what is the correct order?

We measure size in terms of an atom's or an ion's respective atomic or ionic radius. We are aware that atomic radius grows along a group or column of the periodic table and decreases across a period from left to right. This is due to the fact that incomplete electronic shells provide a very poor shield for the nuclear charge and that atomic size depends on the electronic radius. As a result, the atomic size shrinks over time. The atoms: Ba, Cs, Mg, Na are arranged in order of increasing size is $\begin{array}{r}\mathrm{Mg}<\mathrm{Na}<\mathrm{Ba}<\mathrm{Cs}\end{array}$.

Thus the correct option B.