What happens during a reduction reaction?

During a reduction reaction, a substance gains electrons, which is often associated with a gain of hydrogen or a decrease in oxidation state. The term "reduction" primarily refers to the process where electrons are added to an atom or molecule. This is consistent with the concept that when a species undergoes reduction, it typically becomes more negatively charged due to the addition of electrons.

While it may seem that gaining oxygen (or losing hydrogen) supports the reduction definition, in the context of the choices provided, gaining oxygen is not synonymous with a reduction reaction. Instead, the gain of oxygen typically indicates oxidation.

In summary, a reduction reaction primarily involves the gain of electrons, which is not captured by options referring to oxygen gain, hydrogen loss, or proton gain. Thus, gaining electrons is central to the definition of reduction reactions.