What type of airspace requires communication with air traffic control (ATC) before entry?

Class A airspace, which extends from 18,000 feet up to Flight Level 600, requires pilots to have communication with air traffic control (ATC) prior to entering. This airspace is designated for high-altitude jet operations and is closely monitored by ATC to ensure the safety and orderly flow of air traffic.

In Class A airspace, all flights are conducted under Instrument Flight Rules (IFR), necessitating that pilots obtain an ATC clearance before entering. This communication is critical for managing the high volume of aircraft operating in this airspace, ensuring separation and safety at altitudes where navigation options and visibility can be limited.

For other airspace classes, while communication with ATC is also required in certain contexts, it is not universally mandated as it is in Class A. For instance, Class B airspace requires ATC clearance for entry, but only under specific conditions primarily within the vicinity of busy airports. Class C and Class D airspace also require communication with ATC, but this does not necessitate an IFR clearance like in Class A. Hence, Class A is unique in its strict requirement for ATC communication before entry, making it the correct answer.