Which process is characterized by a substance combining to form a single product?

The process in which a substance combines to form a single product is known as a combination reaction. In this type of reaction, two or more reactants come together to create one compound. This can involve elements combining, such as hydrogen gas combining with oxygen gas to form water, or different compounds reacting together to form a new compound. The key feature that distinguishes combination reactions from others is that they result in a single product rather than multiple products.

In contrast, decomposition reactions involve a single compound breaking down into two or more simpler substances; single substitution reactions, also known as single displacement reactions, occur when one element replaces another in a compound; and redox (reduction-oxidation) reactions involve the transfer of electrons between substances, which may not necessarily lead to a single product. Thus, the defining characteristic of a combination reaction makes it the correct answer.