A company is launching a new application that requires a structured database to store user profiles, application settings, and transactional data. The database must be scalable with application traffic and must offer backups. Which solution will meet these requirements MOST cost-effectively?

Amazon Aurora Serverless is the most cost-effective solution because it automatically scales capacity up and down based on demand, meaning you only pay for the resources consumed. This aligns with the requirement for scalability with application traffic and offers built-in automated backups to Amazon S3. The other options are less cost-effective or suitable. Self-managed databases on EC2 (both SQL and NoSQL) incur operational overhead and require manual scaling efforts. While Spot Instances can be cheap, they are not suitable for persistent databases due to their interruptible nature, and Reserved Instances require upfront commitment, which may not be optimal for fluctuating workloads. Amazon RDS is a good option but Aurora Serverless offers superior cost-effectiveness for variable workloads due to its serverless scaling model.