A company moved its on-premises PostgreSQL database to an Amazon RDS for PostgreSQL DB instance. After launching a new product, the database workload increased. The company wants to accommodate the larger workload without adding infrastructure and in the MOST cost-effective way. Which solution meets these requirements?

The most cost-effective solution to handle an increased workload without adding infrastructure is to scale up the existing RDS instance and commit to Reserved Instances. Making the Amazon RDS for PostgreSQL DB instance larger (scaling up) provides more CPU, memory, and I/O capacity to handle the increased load on a single instance. Buying Reserved DB Instances offers significant cost savings compared to On-Demand instances, especially for predictable, sustained workloads like a production database. Making the instance Multi-AZ improves availability and durability but doesn't directly increase read/write capacity for the primary instance to handle a larger workload. Adding another RDS instance (scaling out) would involve more infrastructure and potentially higher management overhead, which contradicts the "without adding infrastructure" constraint. Making it an On-Demand instance doesn't provide cost savings; it's the default pricing model.