Which of the following is quantized according to Bohr's theory of the hydrogen atom?

In the Bohr model of the hydrogen atom,let $R, v$,and $E$ represent the radius of the orbit,the speed of the electron,and the total energy of the electron,respectively. Which of the following quantities is proportional to the quantum number $n$?

The energy of an electron in the ground state of a hydrogen atom is $E$. What is the energy of an electron in the $2^{nd}$ excited state of $Li^{++}$?

Line spectrum was first of all theoretically explained by

Ratio of centripetal acceleration for an electron revolving in $3^{\text{rd}}$ orbit and $5^{\text{th}}$ Bohr orbit of hydrogen atom is

An excited $He^+$ ion emits two photons in succession, with wavelengths $108.5 \, nm$ and $30.4 \, nm$, in making a transition to the ground state. The quantum number $n$, corresponding to its initial excited state is $n = ........$ (for a photon of wavelength $\lambda$, energy $E = \frac{1240 \, eV}{\lambda \, (in \, nm)}$).