Which of the following is NOT considered a reducing agent?

A reducing agent is a substance that donates electrons to another substance in a chemical reaction, thereby reducing that substance while itself being oxidized. Hydrogen peroxide is primarily known as an oxidizing agent under most conditions because it can accept electrons and facilitate oxidation reactions. In various chemical contexts, it often acts by accepting electrons, which is the opposite behavior of a reducing agent.

In contrast, ammonia, manganese, and iron can donate electrons. Ammonia can participate in reduction reactions, manganese can exist in various oxidation states and reduce other species, and iron can donate electrons as well, particularly in ferrous (Fe2+) and ferric (Fe3+) states. Therefore, identifying hydrogen peroxide as not being a reducing agent is accurate, given its predominant role in oxidation processes.