What must a pilot do before entering a Class B airspace under IFR?

Before entering Class B airspace under Instrument Flight Rules (IFR), it is essential for a pilot to obtain clearance from Air Traffic Control (ATC). Class B airspace is designed to prevent congestion and ensure safe operations in busy terminal areas, as it typically surrounds the busiest airports. To maintain an organized flow of air traffic, ATC regulates entry into this airspace by requiring pilots to receive explicit clearance before entering.

This requirement is crucial because it allows ATC to manage aircraft movements safely, ensuring that both IFR and Visual Flight Rules (VFR) traffic can operate without conflicts. Pilots must be aware of their position and the ATC instructions at all times when navigating Class B airspace, making clearance a fundamental aspect of safe IFR operations in these areas.

The other choices do not directly address the immediate requirement for entering Class B airspace under IFR. While conducting a pre-flight inspection, notifying an airport, and filing or updating a flight plan are all important aspects of flight operations, they do not replace the necessity of obtaining clearance from ATC when entering Class B airspace.