The four lines observed in the visible emission spectrum of hydrogen tell us that
a) only certains energies are allowed for the electron in a hydrogen atomb) there are four electrons in an excited hydrogen atomc) the hydrogen molecules they came from have the formula h₄d) we could observe more lines if we had a stronger prism

a) Only certains energies are allowed for the electron in a hydrogen atom

Explanation:

The four lines observed in the visible emission spectrum of hydrogen correspond to the energy of the photons emitted in the transitions of the electrons in the hydrogen atom from one energy level to another one. The energy of these photons corresponds to the difference in energy between the final level and the initial level of the electron.

Since we observe lines, and not a continuous spectrum, it means that only certain energy differences between the levels are allowed: this implies that electrons in a hydrogen atom can only be at certain discrete energy levels.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.