Final answer:
Reduction in chemistry occurs when a substance gains electrons, as described by option A. It is a half of a redox reaction and is paired with oxidation, which is the loss of electrons.
Step-by-step explanation:
In chemistry, reduction refers to the gain of one or more electrons in a chemical reaction. Therefore, the option that describes a situation in which reduction occurs is: A) A substance gains electrons. This process is one half of a redox (reduction-oxidation) reaction, where reduction is paired with oxidation, the loss of electrons. For example, when carbon (C) reacts with oxygen (O2) to form carbon dioxide (CO2), the carbon is oxidized because its oxidation number increases from zero to +4, indicating it has lost electrons. In contrast, the reduction portion of this reaction would involve a different substance gaining those electrons, and hence decreasing its oxidation number.