According to Bohr's theory the radius of electron in an orbit described by principal quantum number n and atomic number Z is proportional to

$Z^2 n^2$ $\frac{Z^2}{n^2}$ $\frac{Z^2}{n}$ $\frac{n^2}{Z}$

$\frac{n^2}{Z}$

$r=\frac{\varepsilon_0 n^2 h^2}{\pi Z m e^2}$;    ∴  $r \propto \frac{n^2}{Z}$