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

Explain covalent radius and van der Waals radius.

Which of the following has the largest size?

$Na^{+}$,$Mg^{2+}$,$Al^{3+}$,and $Si^{4+}$ are isoelectronic. Their ionic size follows the order:

Which ion has the greatest radius among the following?

The statement$(s)$ that are correct about the species $O^{2-}$,$F^{-}$,$Na^{+}$,and $Mg^{2+}$.
$(A)$ All are isoelectronic
$(B)$ All have the same nuclear charge
$(C)$ $O^{2-}$ has the largest ionic radii
$(D)$ $Mg^{2+}$ has the smallest ionic radii
Choose the most appropriate answer from the options given below :