Which of the following phenomena suggests the presence of electron energy levels in atoms?

Due to transitions among its first three energy levels,a hydrogenic atom emits radiation at three discrete wavelengths $\lambda_1, \lambda_2$ and $\lambda_3$ (where $\lambda_1 < \lambda_2 < \lambda_3$). Then,

If the ionization energy of a hydrogen atom in the ground state is $13.6 \ eV$,find the ionization potential of the third orbit in $eV$.

$A$ hydrogen atom initially in the ground level absorbs a photon,which excites it to the $n = 4$ level. Determine the wavelength and frequency of the photon.

The figure shows the energy levels $P, Q, R, S$ and $G$ of an atom,where $G$ is the ground state. $A$ red line in the emission spectrum of the atom is obtained by an energy level transition from $Q$ to $S$. $A$ blue line can be obtained by which of the following energy level transitions?

The diagram shows different transitions across the energy levels for an electron in a certain atom. Among these,which transition represents the emission of a photon with the most energy?