The correct answer is B. Utilizing Amazon DynamoDB to store unique identifiers and checking them in the AWS Lambda function before processing ensures idempotency and handles variable request volumes effectively. DynamoDB is better suited than RDS in this context because IoT devices typically produce unstructured data. Moreover, DynamoDB is a NoSQL service that provides persistent storage, which is crucial for ensuring data consistency and handling retries without losing data. This approach ensures that duplicate requests are managed correctly without the risk of inconsistencies or data loss.