What action must occur for an intersection to remain in flash mode?

To maintain an intersection in flash mode, a reset is typically required to activate the flash mode functionality within the traffic signal controller. This process ensures that the controller transitions from its normal operational mode to a flash state, which is often used during off-peak hours, outages, or special events. The reset action stabilizes the system and confirms that all necessary parameters for flash mode are correctly set up.

While options like power reset or controller update may involve actions that could impact the signal's operation, they do not specifically denote the action required to keep the intersection consistently in flash mode. A manual override, although it could allow a temporary change in operation, does not ensure the signal remains in flash mode without subsequently resetting the system. Therefore, the reset is the most relevant action for the intended function described in the question.