To meet the company's requirements at the lowest cost, the solution must efficiently log all network traffic for Amazon EC2 instances and Amazon RDS databases, retain the information for 12 months, and allow access to metadata including vpc-id, subnet-id, and tcp-flags fields. Option A suggests configuring VPC flow logs with the default fields and storing the logs in Amazon CloudWatch Logs. However, this option does not meet the requirement for including custom format fields like tcp-flags. Option B, configuring Traffic Mirroring, is more complex and costly than necessary for the given requirements. Option D, similar to A, suggests storing logs in Amazon CloudWatch Logs but with additional custom format fields. While this meets the metadata requirement, CloudWatch Logs can become expensive for long-term storage and infrequent access. Option C, configuring VPC flow logs with additional custom format fields and storing the logs in Amazon S3, is the most cost-effective solution. Amazon S3 is designed for long-term storage and infrequent access, making it ideal for retaining logs for 12 months. Additionally, S3's lifecycle policies can be used to transition logs to cheaper storage classes after the first 90 days, further reducing costs. Therefore, Option C is the best choice.