Which of the following best describes the characteristics of polymers?

Polymers are indeed characterized by their composition of repeating units known as monomers. These monomers can be simple molecules that bond together through chemical reactions to form long chains or networks, resulting in the variety of physical properties exhibited by polymers. This structure is fundamental to understanding how polymers function in both natural and synthetic materials. For example, proteins are natural polymers made up of amino acid monomers, and plastics are synthetic polymers derived from various monomers such as ethylene or styrene.

The other options describe characteristics that do not apply to all polymers. Polymers can be branched, cross-linked, or even in a nonlinear arrangement, so they do not always have a linear structure. Furthermore, polymers can be formed from a variety of compounds, not just ionic ones; they can form through covalent bonding. Lastly, polymers can include a wide range of elements, including, but not limited to, carbon and nitrogen; they can contain other elements such as oxygen, sulfur, and chlorine, which add to their diversity and functionality. Thus, the defining characteristic of polymers being composed of repeating monomer units is why the first choice is the accurate description.