The suggested answer is D.
Automatic scaling with min_idle_instances set to 3 is the correct choice for ensuring that the number of instances scales based on request rate while maintaining at least 3 unoccupied instances at all times. Automatic scaling automatically adjusts the number of instances based on various metrics such as request rate, but by setting the min_idle_instances parameter, you guarantee that there will always be a specified minimum number of instances running, even when the request rate is low, thereby fulfilling the requirement of having at least 3 unoccupied instances at any time.