What is the primary feature of a synthesis reaction?

The primary feature of a synthesis reaction is that multiple reactants combine to form a single product. This type of reaction is characterized by the formation of a more complex molecule from simpler building blocks. In a synthesis reaction, the reactants usually include elements or simpler compounds, which then combine chemically to create one compound as a product, illustrating the principle of combination in chemical reactions.

In contrast, when one reactant breaks down into multiple products, that describes a decomposition reaction. The other options also represent different types of reactions; for instance, a single element replacing another in a compound corresponds to a substitution reaction, while a compound reacting with oxygen typically indicates a combustion reaction. Each type of reaction has distinct characteristics, but the key element of a synthesis reaction lies in the combination of multiple reactants.