What scenario would trigger an emergency brake application in ATC systems?

In Automatic Train Control (ATC) systems, an emergency brake application can occur under various scenarios to ensure the safety of the train and its passengers. The correct answer involves a daily test function of ATC car-borne equipment. This is an essential step in routine maintenance and operational checks of the ATC system. During these tests, if the systems detect anomalies or failures, they can autonomously trigger an emergency brake application to prevent potential safety hazards while the train is in operation, thus ensuring that all systems function correctly and safely before normal service resumes.

The other options involve scenarios that would not typically result in a test-triggered emergency brake application. Detection of a passenger emergency could lead to a different type of response specific to onboard safety protocols but does not inherently trigger an emergency application of the brake from the ATC system itself. Similarly, a user command from the train's operator would be a direct command and not an automatic safety feature of the ATC system, and signal loss from track transponders is a critical issue that can lead to system response, but these situations may not automatically trigger an emergency brake application as a functioning ATC would typically manage operations based on its programmed responses to maintain safety unless specified otherwise.