What type of reaction occurs when a substance is oxidized?

When a substance is oxidized, it undergoes a reaction where it loses electrons. This process is foundational in redox reactions where oxidation and reduction occur simultaneously. Oxidation involves the increase in oxidation state of the substance as electrons are removed, which can affect its chemical properties and reactivity.

In the context of redox reactions, every oxidation process must be accompanied by a corresponding reduction, where another substance gains the electrons that the oxidized substance has lost. This transfer of electrons is what drives many chemical and biological processes, including combustion, respiration, and corrosion.

The other options do not accurately reflect the characteristics of oxidation: gaining electrons is the definition of reduction, remaining unchanged describes a non-reactive scenario, and combining with a base does not provide a definition for oxidation in terms of electron transfer. Understanding this fundamental concept is essential for grasping the behavior of substances in various chemical reactions.