Which solution meets these requirements MOST cost-effectively?

Explanations:

Amazon OpenSearch Service is primarily designed for search and analytics use cases rather than straightforward data storage. It may not be the most cost-effective option for storing large amounts of JSON data for backup purposes, especially when quick retrieval is required.

Amazon S3 Glacier is designed for long-term archival storage, where retrieval times can take minutes to hours. Since the data needs to be accessible in milliseconds, this option does not meet the accessibility requirement.

Amazon S3 Standard offers low-latency access and is ideal for frequently accessed data. It allows for quick retrieval (milliseconds) and can easily accommodate the storage of 300 MB of JSON data for 30 days at a low cost.

Amazon RDS for PostgreSQL is a managed relational database service. While it can store JSON data, it is not the most cost-effective solution for simple backup needs and may involve higher operational costs compared to S3 Standard.