What type of reaction occurs when two or more substances combine to form a single product?

A combination reaction, sometimes referred to as a synthesis reaction, occurs when two or more reactants combine to produce a single product. This type of reaction is characterized by the merging of the reactant substances, which can be elements or simpler compounds, into a more complex compound.

For example, when hydrogen gas combines with oxygen gas to produce water, two individual gases come together to form a single liquid product. This is a hallmark of combination reactions, highlighting their nature of merging rather than breaking down or substituting substances.

In contrast, decomposition reactions involve a single compound breaking down into two or more products. Single substitution and double substitution reactions involve the exchange of components between compounds, rather than the straightforward combination to form a single product. Thus, the defining feature of a combination reaction is the formation of one product from multiple reactants, making it the correct choice in this context.