To secure the AWS account root user, the best practices are: (D) Don't create access keys for the root user as it's highly privileged and should not be used for everyday tasks. Instead, create AWS IAM users with minimal required permissions. (E) Enable multi-factor authentication (MFA) for the root user to add an extra layer of security, ensuring that even if the password is compromised, additional verification is required. These practices significantly reduce the risk of unauthorized access to the root account.