What requirements must be met before you enter the airspace represented in D?

The correct answer highlights the importance of two-way radio communications as a primary requirement before entering the airspace represented. In controlled airspace, effective communication with air traffic control (ATC) is critical for ensuring safety and orderly conduct of air traffic. Pilots must establish and maintain two-way radio contact to receive necessary instructions, clearances, and advisories from ATC. This requirement ensures that all aircraft within the airspace are aware of each other's positions and intentions, significantly reducing the risk of in-flight collisions and other incidents.

While transponder equipment and specific visibility requirements may also be necessary for certain airspace classes, the two-way communication aspect is foundational for operating safely within controlled areas. The presence of structured air traffic control systems means that pilots must be able to communicate effectively to navigate and comply with air traffic regulations and operational protocols.