Which of the following describes a polymer?

A polymer is defined as a large molecule made up of repeating structural units called monomers. In this context, the choice that best describes a polymer is a long chain of identical monomers. Polymers can consist of identical monomers, such as in the case of polyethylene, or they can be formed from different types of monomers, such as proteins formed from amino acids. The key characteristic is that they are made up of these repeating units which create a larger and more complex structure.

The other options do not accurately represent what a polymer is. A simple sugar is a monosaccharide, which is a single unit and not a polymer. A protein structure, while it can be a type of polymer, is not described in this case as a simple "polymer", as it does not emphasize the aspect of repeating monomers. Lastly, a single molecule of glucose is also a monosaccharide and similarly does not qualify as a polymer since it is just one unit rather than a chain of connected units. Therefore, the correct answer is clearly the one that captures the essence of what defines a polymer.