Answer: We can complete the sentence as:
"Electrons in atoms exist in particular energy levels. A photon must be emitted or absorbed to move between energy levels" (letter A).
Step-by-step explanation:
The question requires us to choose the best option to complete the sentence: "A ________________ must be emitted or absorbed to move between energy levels".
Energy levels are fixed distances from the nucleus of an atom where electrons can be found. Energy levels can be compared to stairs in a staircase: we can stand on a step, on the previous step or the next step, but never between them; this is what happens to electrons, as they can occupy one energy level or another but not the space between energy levels.
To move from one energy level to another, an electron must absorb energy (to move to a higher level) or emit energy (to move to a lower level). The energy difference between a lower starting level and a higher ending level must correspond to the energy of a photon of light absorbed in order to make the transition between energy levels possible. Additionally, electrons are usually unstable at excited states (when at a higher energy level that it was initially), and the electron quickly returns to the lower energy level emitting the excess of energy in the form of light (as a photon of light).
Therefore, considering the information above, we can complete the sentence as:
"Electrons in atoms exist in particular energy levels. A photon must be emitted or absorbed to move between energy levels" (letter A).