Monomers are best defined as:

Monomers are defined as the basic building blocks or units that chemically bond together to form polymers through a process known as polymerization. These smaller molecules can vary in structure and complexity, but they primarily serve as the foundational elements that link together in long chains, creating larger macromolecules. This relationship is crucial in materials science and engineering, as understanding how monomers come together to form polymers is essential in the development and manipulation of various materials for diverse applications, such as plastics, elastomers, and fibers.

The other definitions provided do not accurately capture the essence of what monomers are. The final products of polymerization would be the polymers themselves, not the individual monomers. Complex structures refer to the resulting polymers or composites formed from the combination of many monomers, but do not pertain to the definition of monomers directly. Lastly, stating that monomers are a type of synthetic plastic is misleading, as monomers themselves can also be derived from natural sources or different materials, and they are not finished products like synthetic plastics.