Which of the following atoms has the smallest atomic size?

Among the ions $O^{2-}, Na^{+}, F^{-}, N^{3-}, Mg^{2+}$,the ions with the smallest and largest radii are respectively:

Assertion $(A)$: $Mg^{2+}$ and $Al^{3+}$ are isoelectronic but the magnitude of the ionic radius of $Al^{3+}$ is less than that of $Mg^{2+}$.
Reason $(R)$: The effective nuclear charge on the outermost electrons in $Al^{3+}$ is greater than that in $Mg^{2+}$.
The correct option among the following is:

How does atomic radius vary in a period and in a group? How do you explain the variation?

Of the following,the ion with the smallest ionic radius is

How does the atomic radius change down a group in the periodic table?