What role should be added to meet the requirements for a RADIUS server?

The appropriate role to be added in order to meet the requirements for a RADIUS server is the Network Policy and Access Services role. This role is specifically designed to enable implementation of Network Policy Server (NPS), which is Microsoft's implementation of a RADIUS server and proxy. NPS provides authentication, authorization, and accounting for network access and is essential for managing network access authentication across various types of devices and users.

The Network Policy and Access Services role includes functionalities such as enforcing network policies, providing RADIUS authentication and authorization for remote access, and managing connection requests from remote access clients. Therefore, adding this role directly supports the deployment and configuration of a RADIUS server environment, facilitating secure network access management.

Other roles listed, such as Remote Access and Routing and Remote Access, are more focused on VPN and dial-up services rather than the specific requirements of a RADIUS server. Active Directory Domain Services, while crucial for overall identity management and authentication within a domain, does not provide the RADIUS functionalities needed on its own without the inclusion of the Network Policy and Access Services role.