A reduction reaction involves either the addition of hydrogen or the removal of:

A reduction reaction is defined by its ability to decrease the oxidation state of a molecule, which typically involves the addition of electrons, hydrogen, or the removal of oxygen. The removal of oxygen during a reduction process lets the molecule gain electrons or hydrogen, effectively reducing its overall charge.

When hydrogen is added, or oxygen is removed, the species undergoes a chemical transformation that lowers its oxidation state. In many organic and biochemical reactions, this process is critical for synthesizing various compounds and facilitating metabolic pathways.

While the other options may have their roles in different chemical processes, they do not directly relate to the definition of a reduction reaction in the context of adding hydrogen or removing oxygen. Carbon and nitrogen do not have a direct effect on the oxidation state in the same manner. Hydrogen peroxide, though an oxidizing agent in some contexts, does not pertain to the standard definition of reduction as described here. Thus, the correct answer reflecting the principles of reduction is the removal of oxygen.