Which reaction type involves the combination of two or more substances to form one product?

The correct choice for identifying the reaction type that involves the combination of two or more substances to form one product is the addition reaction. In an addition reaction, multiple reactants combine to create a single product, which is a hallmark of this type of reaction.

For instance, when hydrogen gas reacts with oxygen gas, they come together to form water, a single compound. This characteristic of combining elements or compounds into a singular product distinguishes addition reactions from other types. In contrast, elimination reactions involve the removal of atoms or groups from a molecule, substitution reactions entail replacing one functional group with another within a molecule, and displacement reactions often involve one element displacing another from a compound. Hence, addition reactions uniquely represent the process of forming one product from two or more reactants.