What is the key characteristic of an oxidation–reduction reaction?

The main feature of an oxidation-reduction reaction is electron exchange.

Explanation:

Every oxeduction reaction is related to an electron transfer between the atoms and / or ions of the reactant substances.

An oxeduction reaction is characterized as a simultaneous process of electron loss and gain, as electrons lost by one atom, ion or molecule are immediately received by others.

For example, a copper sulfate (CuSO4 (aq)) solution is blue due to the presence of Cu2 + ions dissolved in it. If we place a metal zinc plate (Zn (s)) in this solution, over time we may notice two modifications: the color of the solution will be colorless and a metallic copper deposit will appear on the zinc plate.

Therefore, the reaction that occurs in this case is as follows: Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq). Note that there was an electron transfer from zinc to copper. Analyzing separately the transformation that occurred in each of these elements, we have: Zn (s) → Zn2 + (aq). Zinc has lost 2 electrons from metal zinc to cation. In this case, zinc has oxidized. Cu2 + (aq) → Cu (s)  Already with the copper the opposite happened, it gained 2 electrons, from cation copper II to metallic copper. Copper has been reduced.

This explains the two changes observed, as the solution became colorless because the copper ions turned to metallic copper, which settled on the zinc plate. Since there was a simultaneous loss and gain of electrons, this reaction is an example of a redox reaction.

a corroding iron, if that's one of your choices