Which term describes a molecule composed of a series of linked chemical subunits?

The term that describes a molecule composed of a series of linked chemical subunits is "polymer." Polymers are large molecules made up of repeating structural units called monomers, which are covalently bonded together. This linking of many monomers into a chain results in the formation of a polymer, which can have diverse properties and functions depending on the type of monomers used and the nature of their bonding.

In contrast, a monomer refers to a single subunit that can join together to form a polymer, thus not representing a linked structure on its own. A compound is any substance formed when two or more elements are chemically bonded together, but it does not necessarily imply a structure made up of repeating subunits like a polymer does. An element is a pure substance consisting of one type of atom and does not involve the combination of multiple chemical subunits. Therefore, the correct answer captures the essence of a polymer's structure and function in chemistry.