What type of reaction occurs when reactants are combined to produce a more complex product?

The correct choice is synthesis reactions. In synthesis reactions, two or more reactants combine to form a more complex substance. This type of reaction is fundamental in biology, as it underlies important processes such as the formation of macromolecules like proteins, nucleic acids, and carbohydrates from simpler building blocks.

In a synthesis reaction, energy is typically needed to create new bonds between the reactants, resulting in a larger molecule. Such reactions are essential in cellular processes, including growth and repair, as they allow for the construction of complex structures necessary for life.

Other types of reactions listed do not fit this description. Decomposition reactions involve breaking down a complex molecule into simpler components, while exchange reactions consist of swapping parts between molecules rather than building something new. Endothermic reactions refer to energy-absorbing processes but do not specifically describe the combination of reactants to form a more complex product.