A company has multiple AWS accounts in AWS Organizations across global offices. The company must update security group rules to add new office CIDR ranges or remove old CIDR ranges across the organization, while minimizing administrative overhead. Which solution is the MOST cost-effective for centralized management of those CIDR ranges?

The most cost-effective and efficient solution is to use a customer-managed prefix list shared via AWS Resource Access Manager (RAM). A prefix list allows you to group multiple CIDR blocks into a single object. When the CIDR ranges change, you only update the prefix list, and all security groups referencing it automatically reflect the changes. Sharing it with RAM makes it available across all accounts in the organization, centralizing management and minimizing administrative overhead. Creating VPC security groups in the management account would require manual updates in each security group, which is inefficient. AWS managed prefix lists are pre-defined by AWS and cannot be customized for specific office CIDR ranges. AWS Firewall Manager is powerful but often overkill and more complex for just managing CIDR ranges in security groups, and it would still require a mechanism to update the underlying security groups or prefix lists.