What signifies a reduction reaction in a chemical process?

A reduction reaction is defined by the gain of electrons by a reactant. This process can also be understood in terms of oxidation states: when a substance gains electrons, its oxidation state decreases, indicating a reduction. This concept is central to redox (reduction-oxidation) reactions, where reduction and oxidation occur simultaneously; for every reduction reaction, there is a corresponding oxidation reaction in which another substance loses electrons.

In the context of the other options, the loss of protons does not directly indicate a reduction, as it pertains more to acid-base chemistry rather than electron transfer. The formation of a compound from elemental substances involves different types of chemical reactions, such as combination reactions, but does not specifically define reduction. Lastly, sublimation refers to the transition of a substance from a solid to a gas without passing through a liquid phase, which does not involve the concepts of oxidation or reduction. Hence, the option regarding the gain of electrons encapsulates the essence of a reduction reaction most effectively.