The most cost-effective storage strategy that meets the design requirements, according to the file content, is option B. This strategy involves designing the application to store each incoming record in an Amazon DynamoDB table that is properly configured for the scale. Additionally, it leverages the DynamoDB Time to Live (TTL) feature to automatically delete records older than 120 days. This approach is cost-effective because it eliminates the need for manual data deletion processes and utilizes a managed database service that scales with the application's needs. Furthermore, using DynamoDB's TTL feature ensures that storage costs are kept to a minimum by automatically purging old data that is no longer required.