What must be true for a control defined as required to function correctly?

For a control defined as required to function correctly, it must have a specified value for the form to run. This means that when a user is filling out a form, if a required control is present, it must be completed in order for the form to be submitted or processed. This ensures that all necessary information is gathered before proceeding, which is crucial for the integrity of the data collected and for the successful functioning of the system that uses this data.

The other options do not align with the definition of a required control. For instance, a required control cannot be left blank, as the very definition implies it must contain information. Whether or not a control needs a default value can vary, but it doesn't affect its status as required; the essential factor is that the user must provide a value. Lastly, while visibility might help guide users to fill out required fields, required controls do not necessarily need to be visible to function; they simply need to be completed by the user.