Final answer:
An oxidation-reduction reaction is correctly described by the change in the oxidation states of atoms. It involves the movement of electrons where one substance loses electrons (oxidized) and another gains electrons (reduced).
Step-by-step explanation:
The statement that best describes an oxidation-reduction reaction is that this type of chemical reaction exhibits a change in the oxidation states of atoms from reactants to products. An oxidation-reduction reaction, also known as a redox reaction, involves the full or partial transfer of electrons from one reactant to another. One atom or molecule will lose electrons (oxidation), while another gains electrons (reduction).
For instance, in the given example reaction Cu₂O(s) + H₂(g) → 2 Cu(s) + H₂O(g), copper(I) oxide is reduced to copper metal, while hydrogen is oxidized to water. We see here that the oxidation state of copper decreases from +1 in Cu₂O to 0 in metallic copper (Cu), and the oxidation state of hydrogen increases from 0 in H₂ to +1 in H₂O.
It is important to note that the formation of salts and water or precipitates might occur during redox reactions, but these are not defining characteristics of the redox reactions themselves. The essential aspect of a redox reaction is the electron transfer that leads to the change in oxidation state of the elements involved.