Under what circumstance should the receiving controller take immediate action?

Immediate action by the receiving controller is warranted when altitude is not maintained. This situation indicates a potential loss of control or a significant deviation from the assigned flight profile, which can lead to safety concerns such as mid-air collisions or unsafe terrain proximity. Prompt intervention ensures that the aircraft can be safely guided back onto the correct altitude while maintaining separation from other air traffic.

The other choices do not trigger the same level of urgency. A pilot's request may require attention, but it's not an emergency that necessitates immediate action unless context implies otherwise. Acting only during peak traffic hours ignores potentially serious situations that can arise at any time, and waiting for a notification from ground control can delay necessary responses when altitude discrepancies are identified directly by the receiving controller. Therefore, ensuring altitude compliance is critical and requires immediate action to uphold safety in air traffic management.