What determines the oxidizing agent in a redox reaction?

In a redox (reduction-oxidation) reaction, the oxidizing agent is defined as the species that causes oxidation by accepting electrons, and in doing so, it becomes reduced itself. The correct answer is the compound that is reduced, which is appropriate because when a substance gains electrons, it is being reduced, and its reduction allows another substance to be oxidized (which involves the donation of electrons).

Understanding this concept clarifies the essential roles each reactant plays in the overall reaction: the oxidizing agent undergoes reduction, and by doing so, facilitates the oxidation of another compound. Thus, by identifying which substance is being reduced during the reaction, one can correctly determine the oxidizing agent.

The other options do not accurately describe the roles of species in a redox reaction. The compound that donates electrons is referred to as the reducing agent, not the oxidizing agent, and molecular weight does not influence the electron transfer in redox processes. Additionally, gaining protons pertains more to acid-base chemistry than to redox reactions.