Which of the following best describes a polymer?

A polymer is best described as a large molecule composed of repeating structural units. This definition captures the essence of what polymers are made up of: they consist of long chains of smaller units called monomers that are covalently bonded together. These repeating units can vary in structure and complexity, leading to a diverse array of polymers with different properties and functions, such as proteins, nucleic acids, and synthetic plastics. The formation of polymers through processes such as polymerization allows for the creation of materials with specific physical and chemical characteristics, making them crucial in both biological systems and industrial applications.

The other choices do not accurately represent the concept of polymers. Small molecules serve as the building blocks of polymers, while complex carbohydrates like starches refer to specific types of polymers (polysaccharides) but do not encompass the broader definition of a polymer. Enzymes, while important in biological reactions, are proteins that can be considered a type of polymer, but they are not defined by the repeating structural units characteristic of all polymers.