What is a polymer?

A polymer is defined as a large molecule that is composed of repeating structural units, typically connected by covalent chemical bonds. These repeating units can be simple molecules known as monomers, which can link together through polymerization processes to form complex structures.

Polymers can vary greatly in their properties and applications, ranging from everyday materials like plastics and rubber to natural substances like proteins and DNA. The key characteristic of a polymer is its large size and repetitive structure, which gives it distinct physical and chemical properties compared to smaller or simpler molecules.

The other options describe substances that do not encompass the defining features of polymers. For instance, a simple small molecule is fundamentally different from a polymer since it lacks the large size and repetitive nature that defines polymers. A compound that can only exist in one state may refer to specific substances that do not undergo changes in their physical state; this does not apply to polymers, which can be found in various forms depending on conditions. Lastly, a type of crystalline solid does not accurately describe polymers, as many polymers can be amorphous, rather than exclusively crystallized. Therefore, the most accurate definition of a polymer in this context is that it is a large molecule formed from repeating units.