In which type of reaction do products form more complex substances from simpler ones?

The correct answer is that the reaction forming more complex substances from simpler ones is characterized as a combination reaction. In these chemical processes, two or more reactants combine to form a single product, resulting in a more complex molecular structure.

This type of reaction is fundamental in building larger molecules from simpler atomic or molecular units. An example of a combination reaction is the synthesis of water from hydrogen and oxygen gases:

[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) ]

In this equation, the elemental gases combine to create the more complex liquid water.

The other types of reactions serve different purposes: decomposition reactions break down compounds into simpler substances, replacement reactions generally involve one element taking the place of another element in a compound, and displacement reactions specifically refer to a subset of replacement reactions where an element displaces another in a compound.

Thus, combination reactions are specifically noted for their role in constructing more complex products from simpler reactants.