What piece of equipment should a pilot use to inform ATC of any significant delays upon arrival?

Using a transponder is essential for maintaining communication with Air Traffic Control (ATC). The transponder is a device installed in the aircraft that helps identify it on radar. While the transponder primarily aids in tracking and identifying aircraft, pilots can also use it to relay certain information, such as their current location and status.

In the context of informing ATC of significant delays upon arrival, it is common for pilots to be instructed to keep ATC updated with their estimated arrival times and to report changes in those estimates. However, the most effective and immediate method for doing this, particularly while approaching an airport, is through a reliable communication channel such as a transponder that includes Automatic Dependent Surveillance-Broadcast (ADS-B) capabilities, allowing real-time updates to air traffic controllers.

The other options may not provide the appropriate means for timely communication with ATC. Handheld radios, while they can be used, may not be as effective as onboard systems. Mobile phones are not typically used for operational communications in-flight due to regulations and the need for direct ATC contact. Visual signals are limited in their effectiveness and depend on visual contact between the aircraft and ground personnel. Thus, the transponder stands out as the most suitable and efficient equipment for this particular