What action must pilots take when flying in controlled airspace under IFR?

When flying in controlled airspace under Instrument Flight Rules (IFR), the primary requirement is for pilots to communicate with Air Traffic Control (ATC) and adhere to their instructions. This ensures safety and efficiency within controlled airspace, where ATC manages aircraft movements to prevent collisions and maintain organized traffic flow. Communication with ATC allows pilots to receive important information such as clearances, navigation instructions, and weather updates, all of which are crucial for safe IFR operations.

The other options do not align with the requirements of IFR operations in controlled airspace. For instance, flying below the minimum altitude is unsafe and violates regulations that set essential safety margins. Not filing a flight plan contradicts the IFR protocol, as pilots are obligated to file a flight plan to ensure ATC is aware of their intended route and has the opportunity to manage air traffic effectively. Avoiding communication with ATC would also contravene the requirements of IFR, as it is critical for maintaining situational awareness and coordination in busy airspace. Thus, effective communication and compliance with ATC instructions are fundamental aspects of IFR operations in controlled airspace.