Auto-scaling is the correct approach for handling unexpected surges in traffic because it allows resources to be automatically adjusted based on current demand. This means that during peak times, additional resources will be allocated to handle the surge of users, and these resources will be scaled back down during low-usage periods, maintaining cost-efficiency and service availability. Horizontal scaling refers to adding more instances to handle more load but does not imply automatic adjustment. Vertical scaling involves adding more power to existing instances rather than adjusting instance numbers and may require downtime. Cloud bursting is a method of using public cloud resources to handle excess load but is not typically an automatic process.