Which solution will meet these requirements MOST cost-effectively?

Explanations:

Adding read replicas can improve read scalability but does not address the issue of write-heavy loads on the primary database instance. Purchasing Instance Savings Plans and RDS Reserved Instances may reduce costs but does not provide the required scalability for fluctuating workloads.

Migrating to an Aurora DB cluster with multiple writer instances can increase write capacity, but this solution incurs higher costs without efficiently handling sudden changes in load. Instance Savings Plans may help with costs but do not address scalability needs.

An Aurora global database improves cross-region replication but is unnecessary for handling fluctuating traffic within a single region. This option is also likely to be more expensive and does not focus on the write-heavy nature of the application. Compute Savings Plans and Reserved Instances would not optimize cost effectively for variable workloads.

Migrating to Aurora Serverless v1 allows the database to automatically scale capacity up or down based on demand, making it cost-effective for variable workloads. This option minimizes costs during periods of low usage and scales efficiently during traffic spikes. Purchasing Compute Savings Plans may help with costs, but Aurora Serverless’s inherent ability to adjust based on load is the key benefit here.