How are airspaces classified from A to G?

Class A airspace is indeed the most controlled airspace, where operations require adherence to strict regulations and flight under instrument flight rules (IFR) is mandatory. This classification is primarily focused on ensuring the safety of aircraft flying at high altitudes, where they can operate above the weather and avoid conflicts with other air traffic.

On the other hand, Class G airspace is classified as uncontrolled airspace. This type of airspace typically encompasses lower altitudes where pilots can operate under visual flight rules (VFR) without the necessity of air traffic control clearance, allowing for more freedom and flexibility in operations.

Thus, the correct option effectively captures the hierarchical structure of airspace classification, which ranges from the highly regulated Class A down to the uncontrolled Class G. Additionally, the incorrect options misrepresent the nature and regulations surrounding each class, highlighting the importance of understanding the distinction between controlled and uncontrolled airspace in aviation.