The correct approach is to set the min_idle_instances property in the app.yaml file, as this ensures a specified number of instances are always running and ready to serve traffic, which is crucial for handling sudden load spikes. This method is effective for priming your application for rapid increases in request volume, though it's important to note that these instances incur charges regardless of traffic. Options A and D are incorrect because manual scaling does not guarantee minimal running instances. Option B is not suitable as min_instances pertains to dynamic scaling and does not ensure idle instances are available for burst traffic.