What is a characteristic of controlled airspace?

In controlled airspace, one of the key characteristics is that it requires pilots to establish communication with air traffic control. This ensures that aircraft can operate safely and efficiently within that airspace, as it allows for the management of traffic and helps prevent collisions. Air traffic control provides crucial information, such as traffic advisories, weather updates, and other pertinent data that aids pilots in navigating safely.

While other options mention aspects of airspace, they do not accurately describe the nature of controlled airspace. For instance, unrestricted flight and availability limited to VFR operations are not defining features of controlled airspace, as it encompasses various operation types and altitudes. Additionally, the assertion that there are no restrictions for navigation misrepresents the structured nature of controlled airspace, which is designed to maintain safety through communication and oversight.