How can a redox reaction be identified?A.Oxidation states have changed.B.Atoms have rearranged.C.Protons have been transferred.D.Ions are present in solution.

A redox reaction can be identified by checking if the oxidation states of the atoms have changed. In a redox reaction, one or more electrons are transferred between atoms, which results in a change in their oxidation states. This is option A: "Oxidation states have changed."

While atoms may rearrange in a redox reaction (option B), this is not a definitive indicator of a redox reaction as atoms also rearrange in many other types of chemical reactions.

Protons being transferred (option C) is characteristic of an acid-base reaction, not a redox reaction.

The presence of ions in solution (option D) can occur in many types of reactions and is not specific to redox reactions.

So, the correct answer is A: "Oxidation states have changed."