A is the correct answer because it uses AWS Directory Service for Microsoft Active Directory to join the Windows EC2 instances to an Active Directory domain on AWS and enable MFA. AWS Directory Service for Microsoft Active Directory, also known as AWS Managed Microsoft AD, is a fully managed service that is powered by Windows Server 2019. It allows you to run directory-aware workloads in the AWS Cloud, including Microsoft SharePoint and custom .NET and SQL Server-based applications. You can also configure a trust relationship between AWS Managed Microsoft AD in the AWS Cloud and your existing on-premises Microsoft Active Directory. AWS Managed Microsoft AD supports MFA by integrating with your existing RADIUS-based MFA infrastructure. To join the Windows EC2 instances to an Active Directory domain on AWS, you can use an Amazon Workspace, which is a fully managed, secure desktop computing service that runs on AWS. You can connect to and use the Workspace for domain security configuration tasks. References:

https://docs.aws.amazon.com/directoryservice/latest/admin-guide/directory_microsoft_ad.html

https://docs.aws.amazon.com/directoryservice/latest/admin-guide/ms_ad_join_instance.html

https://docs.aws.amazon.com/workspaces/latest/adminguide/amazon-workspaces.html