Radius of the first orbit of the electron in a hydrogen atom is 0.53 Å. So, the radius of the third orbit will be

2.12 Å 4.77 Å 1.06 Å 1.59 Å

4.77 Å

$r_n \propto n^2 \Rightarrow \frac{r_3}{r_1}=\frac{3^2}{1} \Rightarrow r_3=9 r_1=9 \times 0.53=4.77 ~Å$