Final answer:
To make a custom entity available in the interactive service hub, you should modify the setting 'Enable for interactive experience'. This allows the entity to be used within the streamlined interface of Dynamics 365's interactive service dashboard.
Step-by-step explanation:
If you need to ensure that a custom entity is available from the interactive service hub in your organization, the setting you should modify is C. Enable for interactive experience. The interactive service hub is designed to provide a more streamlined user interface for working with records in Dynamics 365, and enabling this setting for an entity allows it to be used within this interface. This setting can be found in the entity's customization options and will make the entity appear in the interactive dashboard experiences.