If the radius of the first Bohr orbit of hydrogen atom is 0.53 Å, the radius of the third orbit of a hydrogen atom is .

The correct option is A 4.77 Å

Try to recall the formula for radius of nth orbit from Bohr's postulates.

$r_n$ = $r_1$ ($n^2$)

= $r_1$ ($3^2$) Where $r_1$ is the radius corresponding to the ground-state $n=1$

= $0.53$ $\times$ $9$ = $4.77\AA$