EKS lets teams “run Kubernetes on AWS without needing to install and operate your own Kubernetes control plane,” and with AWS Fargate, you “run Kubernetes pods without managing servers,” reducing operational overhead for worker nodes. For the data layer, Amazon DocumentDB (with MongoDB compatibility) “supports the MongoDB API and drivers,” allowing existing MongoDB applications to work without application changes, while the service “automatically scales storage,” provides “high availability across multiple Availability Zones,” automatic backups, and patching. Because the company cannot change code or deployment methods, keeping Kubernetes (EKS) and the MongoDB API (DocumentDB compatibility) is essential. Options A and C either change the orchestrator (to ECS) or the database API (to DynamoDB), which would require code/deployment changes. Option A also leaves you managing EC2 nodes and a self-managed MongoDB on EC2, increasing ops burden. Therefore, EKS on Fargate + Amazon DocumentDB minimizes operations and preserves compatibility.