Between $Al$ and $Al^{3+}$,which has a smaller radius and why?

(B) $Al^{3+}$ has a smaller radius. This is because $Al$ is the parent atom of $Al^{3+}$,and $Al^{3+}$ is formed by the removal of $3$ electrons from $Al$. Consequently,the effective nuclear charge increases in $Al^{3+}$,which results in a smaller size compared to the neutral $Al$ atom.