Atomic Structure - Chemistry

what does the question mean by repulsion, is it between atoms or just between electrons? can you explain please? the answer is D.

Hey Lara, in chemistry and physics you should remember that electrons are always looking for the most energetically stable sate (or favorable state, which is easier to remember).
Electrons prefer to join the lowest orbital available for them as they have the least amount of repulsion from other bodies in that area (mostly other electrons)

Remember that orbitals is just a very simplified visual representation of where the electrons are most likely to be, but the electrons can be anywhere around the atom.
If you think about it logically, the more electrons you add over time, the new electron won’t ‘want’ to be near other electrons, with the same negative charge as them, to do so they will first join the lowest energy level available orbital because it is the most stable.

To the question itself: the energy from the repulsion (of the other electron in the orbital) is less than the energy required for the electron to jump to a higher level - which means that the energy the electron “needs to spend” to join another orbital that already has one electron is less than the energy that electron will have to “spend” to join an empty, but higher-level orbital.