In Bohr's model of hydrogen atom, the ratio of periods of revolution of an electron in $n=2$ to $n=1$ is

The correct option is C $8:1$
According to Bohr's atom model,

$T\propto \frac{n^3}{Z^2}$

For Hydrogen atom $Z=1$,

$\Rightarrow T\propto n^3$

$\Rightarrow \frac{T_1}{T_2}={\left(\frac{n_1}{n_2}\right)}^3={\left(\frac{1}{2}\right)}^3$

$\Rightarrow \frac{T_2}{T_1}=\frac{8}{1}$

Hence, option $\left(C\right)$ is correct.

Why this question? This question is based on the concept of time period of electron in an orbit.