Auto-scaling is the correct answer because it enables a cloud environment's compute resources to automatically scale up or down based on the current demand. This is particularly useful for scenarios like sales events, where traffic can unpredictably surge, and manual scaling might not be timely or efficient. Horizontal scaling involves adding more instances of the same type, which is what auto-scaling does. Vertical scaling, while sometimes useful, involves adding more power (CPU, RAM) to the existing instances, which has its limits and can't be done on-the-fly as easily as horizontal scaling. Cloud bursting could handle extra load, but it is a more complex solution that involves spilling over into another cloud provider's space, rather than scaling within the same environment; usually, this would be used when a private cloud's resources are temporarily insufficient.