What type of reaction involves the gain or loss of electrons?

A reaction that involves the gain or loss of electrons is categorized as a redox reaction, which stands for reduction-oxidation reaction. In these types of reactions, one substance undergoes oxidation, meaning it loses electrons, while another substance undergoes reduction, meaning it gains those electrons. This transfer of electrons often accompanies energy changes and can be observed in various biological and chemical processes, such as cellular respiration and photosynthesis.

While other types of reactions, such as exchange, substitution, and decomposition, involve the rearrangement of molecules or compounds, they do not fundamentally focus on the transfer of electrons. Specifically, exchange reactions typically involve switching of partners in a reaction without a direct electron transfer; substitution reactions involve one element being replaced by another in a compound; and decomposition reactions break a single compound into simpler substances without the characteristic electron transfer of redox processes. Thus, the defining feature of redox reactions is the electron transfer, making it the correct choice in this context.