Which of the following is a key pre-requisite required to ensure MFA can be used along with AWS Workspaces

Answer options:

A.A RADIUS Server deployed in the on-premise environment
B.An MFA Server deployed in the on-premise environment
C.An MFA Server deployed in AWS
D.An MFA Server deployed in AWS and in the on-premise environment

Answer - A
To enable MFA for AWS services such as Amazon WorkSpaces and QuickSight, a key requirement is an MFA solution that is a Remote Authentication Dial-In User Service (RADIUS) server or a plugin to a RADIUS server already implemented in your on-premises infrastructure. RADIUS is an industry-standard client/server protocol that provides authentication, authorization, and accounting management to enable users to connect network services. 
For more information on enabling MFA for AWS Workspaces, please refer to below URL:
https://aws.amazon.com/blogs/security/how-to-enable-multi-factor-authentication-for-amazon-workspaces-and-amazon-quicksight-by-using-microsoft-ad-and-on-premises-credentials/