The four most suitable applications for ECS are: A. Official corporate websites: ECS can effectively host corporate websites, providing scalability, performance, and reliability for business-critical web applications. B. Large-traffic apps: ECS is well-suited for handling high-traffic applications that require dynamic scaling to accommodate fluctuating user demands. It can efficiently distribute traffic across multiple instances to ensure optimal performance. D. Simple web applications: ECS is a versatile platform that can host simple web applications, offering flexibility and cost-effectiveness for basic web deployments. E. No limited online storage: ECS can be integrated with Alibaba Cloud Object Storage Service (OSS) to provide unlimited online storage for your web applications and data. While ECS can be used for databases, it's not the primary purpose or recommended solution.