Which of the following is an example of a functional region?

A functional region is defined by a specific set of activities or interactions that occur within it, often centered around a node or focal point. In the case of Washington D.C., it serves as the political center of the United States where governmental functions, activities, and interactions are concentrated. The presence of federal agencies, legislative sessions, and other political institutions embodies the operational essence that characterizes a functional region.

The other options illustrate different types of regions. The Sahara Desert represents a formal region defined by its physical characteristics and climate; the Rocky Mountains is a physical region defined by geographical features; and the Great Lakes, while significant in terms of water bodies, fall under a natural or physical classification rather than one based on function and human activities. Thus, Washington D.C. is distinctly identified as a functional region due to its role in facilitating government and governance-related interactions.