Final answer:
By default, if no user is established for Knowledge, access is generally restricted and not available to everyone. Typically, users with an internal role would have access, depending on the system's data sharing configurations.
Step-by-step explanation:
If no user is established for Knowledge, by default the access is typically restricted to those within the organization or individuals who have been given specific roles that have the permissions to access Knowledge bases. However, it's necessary to define specific permissions, and if this is not done, the default access can vary depending on the system being used. In Salesforce, for instance, if no user is specifically given access to Knowledge, its default state is restricted and thus it would not be accessible to everyone (option A), external users (option D), or customers (option B) without additional configuration. Instead, generic internal Salesforce users – those with an internal role – would typically have access by default (option C), since Salesforce's default internal data sharing model tends to be more permissive within an organization.