A company runs a workload in AWS. Which AWS best practice most directly helps ensure the architecture is as cost-effective as possible?

Rightsizing is the most direct best practice for cost-effectiveness. It involves continuously monitoring resource utilization and adjusting the size and type of AWS resources (like EC2 instances or RDS databases) to match actual workload needs. This prevents over-provisioning, which leads to unnecessary costs, and under-provisioning, which can cause performance issues and potential re-work. Loose coupling, while a good architectural practice for resilience and scalability, doesn't directly address cost-effectiveness as its primary goal. Caching improves performance and can indirectly reduce costs by lowering the load on primary resources, but rightsizing is about the fundamental cost of the resources themselves. Redundancy is crucial for high availability and fault tolerance, but it typically increases cost rather than directly reducing it, though it provides value for resilience.