Question

which of these statements describes bohr’s theory of the hydrogen atom correctly?
energy, in the form of radiation, must be continually supplied to keep the electron orbiting.
the electron can exist at specific energy levels and move between them by gaining or losing energy.
the emission spectrum of hydrogen can have only one line because it has only one electron.
the electron can move from one energy level to another without gain or loss of energy.

Explanation:

Bohr's model of the hydrogen atom posits that electrons occupy fixed, discrete energy levels (stationary states) around the nucleus. Electrons do not emit energy while in these states, and they transition between levels by absorbing or emitting photons (energy) equal to the difference in energy between the levels. This explains the discrete emission spectrum of hydrogen (multiple lines, not one), and no continuous energy supply is needed for stable orbits.

Answer:

The electron can exist at specific energy levels and move between them by gaining or losing energy.