Which AWS service is a key-value database that provides sub-millisecond latency on a large scale?

The correct answer is A because Amazon DynamoDB is a key-value database that provides sub-millisecond latency on a large scale. Amazon DynamoDB is a fully managed, serverless, and scalable NoSQL database service that supports both key-value and document data models. The other options are incorrect because they are not key-value databases. Amazon Aurora is a relational database that is compatible with MySQL and PostgreSQL. Amazon DocumentDB (with MongoDB compatibility) is a document database that is compatible with MongoDB. Amazon Neptune is a graph database that supports property graph and RDF models. Reference: Amazon DynamoDB FAQs