What is a polymer?

A polymer is best defined as a long chain formed from repeating monomer units. This description highlights the key characteristics of polymers, which consist of many identical or similar structural units (monomers) bonded together in a specific arrangement. This repeated structure can result in very large molecules, which is a defining trait of polymers.

The formation of polymers through processes like polymerization involves linking together these smaller monomers, often resulting in materials with unique properties that differ significantly from those of their individual monomer units. Examples include polyethylene and polystyrene, which are common polymers formed from the polymerization of simple moleculard monomers.

Understanding this concept is essential as it lays the foundation for studying various polymers' chemical and physical properties, uses, and behaviors in different contexts, such as in manufacturing and biology. The other options provided do not accurately define what a polymer is, focusing instead on oversimplified or unrelated structures.