To restrict access to S3 buckets so that only IAM principals from within the AWS Organization can access them, an SCP (Service Control Policy) with conditions using the aws:PrincipalAccount or preferably aws:PrincipalOrgID can be applied at the OU level.

SCPs restrict the maximum permissions for IAM entities in member accounts and can be used to enforce access control policies across accounts.

The aws:PrincipalAccount condition restricts access to principals from specific accounts, while aws:PrincipalOrgID restricts based on the organization ID.

IAM permissions boundaries (Option B) cannot be applied organization-wide and do not enforce restrictions across accounts.

AWS RAM (Option C) is for sharing resources but does not restrict S3 bucket access based on organizational principals.

There is no such thing as an RCP in AWS Organizations (Option D is invalid).