The radius of which ion is closest to that of $Li^{+}$ ion?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: The ionic radii of $O^{2-}$ and $Mg^{2+}$ are same.
Reason $R$: Both $O^{2-}$ and $Mg^{2+}$ are isoelectronic species.
In the light of the above statements,choose the correct answer from the options given below:

Which element has a larger atomic size compared to oxygen $(O)$?

$Al^{3+}$ has a lower ionic radius than $Mg^{2+}$ because

The ionic radius is:

Which of the following ions has the smallest radius?