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

To explain his theory,Bohr used

The magnetic field at the centre due to the motion of an electron in the first Bohr orbit is $B$. The magnetic field due to the motion of an electron in the second Bohr orbit at the centre will be

In a hydrogen atom,let $R$,$v$,and $E$ denote the radius,velocity,and total energy of the electron,respectively. Then,the principal quantum number $n$ is proportional to which of the following?

When an electron in a hydrogen atom is excited from its $4^{th}$ to $5^{th}$ stationary orbit,the change in angular momentum of the electron is (Planck's constant: $h = 6.6 \times 10^{-34} \ J \cdot s$)

The energy equivalent to the Rydberg constant is ...... $eV$.