Which of the following options describes a function of Whois?

The function of Whois is best described by the option that provides ownership and registration details of a domain. Whois databases contain information about registered domain names, including details such as the domain owner's name, contact information, registration dates, and the domain's status. This service is essential for identifying who is behind a domain, which can be useful for resolving disputes, verifying the legitimacy of sites, and obtaining contact information for domain administration purposes.

In contrast, the other options refer to functions that are not associated with Whois. Displaying server speed statistics is not part of Whois services, as that information pertains more to web performance monitoring. The encryption level of a website relates to SSL certificates and security measures, which are different from the registration data managed by Whois. Lastly, listing all active users of a domain does not align with the purpose of Whois, as it focuses on domain ownership rather than user activity.