Which of the following might trigger a reset prompt on the CDU?

A reset prompt on the Control Display Unit (CDU) typically occurs when the system identifies a fault or a mode that requires user intervention to restore normal operation. In this context, SPDS stands for System Performance Data System, which is critical for monitoring the health and performance of various aircraft systems. If the SPDS enters failsafe mode, it indicates that the system has detected an issue that may impair its function or the overall safety of the aircraft. This situation necessitates a prompt to ensure that the crew can take appropriate action.

The other scenarios, while serious in their own right, do not directly result in a reset prompt for the CDU. A landing gear malfunction may trigger alarms or indications but does not specifically engage a reset of the CDU. Engaging the autopilot and exceeding altitude settings might lead to alerts, but these do not typically necessitate a reset of the system, as they are operational parameters rather than fault states triggering the CDU's response. Therefore, encountering the SPDS in failsafe mode is indeed the most likely scenario to prompt a reset command on the CDU.