Final answer:
Authenticated access to a Force.com site can be provided through the integration with an active Customer Portal or an active Partner Portal. These provide a structured way for users to log in securely.
Step-by-step explanation:
The mechanism that allows for authenticated access to a Force.com site is an active Customer Portal or an active Partner Portal. When you create a Force.com site, it is public by default, but you can enable authentication by integrating it with either a Customer Portal or a Partner Portal. These portals provide a framework for creating authenticated sessions, allowing registered users to log in and interact with the application in a secure manner. It is important to note that setting restricted IP ranges for your site does not provide authentication, but rather an additional layer of security that only allows users from specified IP addresses to access the site.