Which of the following has the smallest size?

The ions $S^{2-}, Cl^{-}, K^{+}, Ca^{2+}$ are isoelectronic. Their ionic radii show

The reason for a cation to be smaller than its parent atom is

What is the method to estimate the volume of an individual atom?

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:

What happens to the atomic radius of $p$-block elements as we move from left to right across a period in the periodic table?