Which best describes a monomer?

A monomer is accurately described as a relatively small molecule that can join together to form a polymer. Monomers are the basic building blocks of polymers; they typically possess reactive functional groups that allow them to link together in a process called polymerization. When multiple monomers chemically bond through various types of reactions, they create long chains known as polymers, which exhibit unique properties and functions compared to their monomer components. This definition encapsulates the fundamental role of monomers in chemistry, particularly in the study of macromolecules.

The other descriptions provided do not align with the concept of a monomer. For instance, a large complex molecule refers to a polymer rather than a monomer. A reactive intermediate may be part of a polymerization process, but it does not accurately define what a monomer is. Finally, while there are chemical reactions involving monomers, this option does not pinpoint the essence of a monomer itself, which is primarily its role as a small molecule in the formation of polymers.