Elastic scale-out is used to increase data-plane capacity by placing a Virtual Service on additional Service Engines or by increasing Service Engine capacity in response to demand. Avi Elastic HA combines scale-out performance and high availability, and Avi autoscale features are designed to add Service Engine capacity when traffic growth requires more resources. This directly addresses situations where a Virtual Service must handle heavy increases in traffic volume without application disruption. Controller failure protection is handled by Controller clustering, not elastic data-plane scale-out. Processing multiple protocols is a Virtual Service design and profile configuration concern. Enabling WAF is an application security feature and can increase resource needs, but it does not itself define the scenario requiring elastic scale-out. Therefore, the correct answer is C.