Which AWS service provides this functionality with the LEAST operational overhead?

Explanations:

Amazon RDS for PostgreSQL requires management of database instances, scaling, and maintenance tasks, leading to higher operational overhead compared to serverless options.

Amazon DynamoDB is a fully managed, serverless NoSQL database that automatically scales and has minimal operational overhead, making it ideal for storing data from a recommendation engine.

Amazon Neptune is a graph database service that may not be necessary for a recommendation engine unless the data model specifically requires graph processing, and it also has more operational overhead than DynamoDB.

Amazon Aurora is a managed relational database that requires some level of management and scaling, which adds operational overhead compared to fully serverless solutions like DynamoDB.