Which monomer serves as the building block of glycogen, a polymer made up of many glucose molecules?

The correct answer is glucose.

Glycogen is a polysaccharide composed of multibranched glucose units (about 8-12 glucose units) linked by the glycosidic bonds. It is a form of energy storage in the body, especially in the liver cells in the form of granules. When there is a sudden need for glucose, glucose from the glycogen can be rapidly mobilized. It also plays an important role in the glucose cycle.