Which database solution meets these requirements at the LOWEST cost?

Explanations:

Amazon Neptune is optimized for graph-based data, suitable for use cases like knowledge graphs, social networks, or recommendation engines. It is not designed for high-throughput transactional workloads or simple key-value access patterns, making it costly and inefficient for this scenario.

Amazon Aurora PostgreSQL Serverless can handle transactional workloads and offers high availability but may not meet the cost efficiency or millisecond latency requirements, especially under extremely high request rates, and has more operational complexity than DynamoDB.

Amazon RDS for PostgreSQL provides durability and availability but lacks the auto-scaling and operational simplicity required for high-throughput, low-latency, cloud-native applications. It would also require more maintenance than DynamoDB.

Amazon DynamoDB is a fully managed, serverless, key-value database designed to scale for high-throughput workloads with minimal operational overhead. It meets the requirements for low latency, high availability, and cost-effectiveness for handling billions of requests daily.