Which of the following indicates that a train defect has been detected?

A stop signal being received is the correct indication that a train defect has been detected. In railway operations, stop signals are specifically designed to alert train crews to immediate concerns that may include safety issues such as defects detected on the train or tracks. When a stop signal is encountered, it typically requires the train to halt, allowing for an inspection or immediate action to address any potential danger. This is a critical safety measure to ensure the train's integrity and the safety of its passengers and cargo.

In contrast, when a train is moving at full speed, it suggests that there are no immediate issues detected. Crew members reporting no issues further indicates that everything is functioning properly. Similarly, a train passing without inspection implies that it was cleared to continue its journey without any indication of defects. These scenarios do not provide any warning or signal of potential defects, highlighting the importance of stop signals in railway safety protocols.