Which characteristic defines a functional region?

A functional region is characterized by its organization around a central node or hub, with surrounding areas linked to this center through various interconnections, such as transportation, communication, or economic activities. The defining feature of a functional region is not just the presence of a central point, but the relationships and interactions that emanate from it, influencing how the region operates as a cohesive unit.

In contrast, other options describe different types of regions. A common cultural identity refers to a vernacular region, while a distinct physical boundary pertains more to formal regions, defined by clear, measurable traits. Although economic activity can be a part of a functional region's interactions, the region is not solely based on economics, which makes it distinct from the economic focus suggested in that option. Thus, the correct understanding of a functional region revolves around the connections and interactions around a central point rather than merely cultural, physical, or purely economic criteria.