What feature is essential for enabling specific Internet access settings on a Client Access server?

The essential feature for enabling specific Internet access settings on a Client Access server is the External URL. This configuration defines the address that clients use to access Exchange services over the Internet. It ensures that clients can connect seamlessly to Exchange services, such as Outlook on the web or ActiveSync, using a URL that is externally accessible.

Setting the External URL allows the Client Access server to respond correctly to requests coming from the Internet, providing users with appropriate connectivity while maintaining security and functionality. In scenarios where different services are hosted on the same server, defining distinct external URLs for specific services is important for proper routing and access.

The other choices, while relevant to certain functionalities within the Exchange infrastructure, do not directly pertain to enabling Internet access settings specifically. External bindings relate to the network configurations on the server itself, client certificates deal with authentication and encryption but are not necessary for the configuration of Internet-facing URLs, and access tokens pertain to authorization and session management rather than the specific access configuration of the services on the Client Access server.