If an electron is moving in the $n^{\text{th }}$ orbit of the hydrogen atom, then its velocity $\left(v_n\right)$ for the $n^{\text{th }}$ orbit is given as:

In the given figure, the energy levels of hydrogen atom have been shown along with some transitions marked $A,B,C,D$ and $E$.
The transitions $A,B$ and $C$ respectively represent