What is a polymer?

A polymer is defined as a very large molecule that is composed of many smaller repeating units known as monomers. These monomers are covalently bonded together in a chain-like formation, resulting in a structure that can be extremely large and complex. Polymers play a crucial role in both nature and industry, serving as the basis for many materials such as plastics, DNA, proteins, and natural rubber.

The correct answer is rooted in the understanding of how polymers are formed through polymerization—a chemical process that links small molecules into larger structures. The size and properties of polymers can vary widely based on the nature of the monomers and the way they are linked together.

The other choices do not accurately capture the nature of a polymer. A small molecule that cannot bond describes a different type of compound and doesn't encompass the essence of polymer formation. A type of chemical reaction does not specifically define what a polymer is; while polymers are formed through reactions, this answer does not give the complete picture. Lastly, describing a polymer as a mixture of various chemical compounds misrepresents it, as polymers are specific large molecules formed from repeated units rather than a combination of different compounds.