What should the solutions architect do to meet these requirements?

Explanations:

Enabling DynamoDB Accelerator (DAX) can improve read performance by caching frequently accessed data, but it does not address cost-effectiveness for variable workloads or help manage scaling during predictable traffic peaks and declines.

Multi-AZ replication improves availability and durability but does not directly address the cost-effectiveness or scaling needs associated with fluctuating traffic. It adds redundancy without directly managing workload variations.

Enabling DynamoDB auto scaling allows the database to automatically adjust its read and write capacity based on the actual traffic load, providing a cost-effective solution that aligns with the predictable peaks and valleys of website traffic.

DynamoDB On-Demand capacity is ideal for unpredictable workloads as it scales automatically to handle traffic. However, since the traffic is predictable, auto scaling is a more cost-effective approach, allowing for better management of provisioned capacity based on expected demand.