Which device is used to alter the volume of air passing through a confined cross-section?

The correct choice is a damper because it is specifically designed to control the flow of air within a duct system or similar confined space. Dampers can open or close to adjust the volume of air passing through, thereby regulating airflow and ensuring balanced ventilation in a system. They play a crucial role in enhancing comfort, improving energy efficiency, and ensuring that systems operate effectively.

In contrast, a vent is primarily an outlet designed for the passage of air but does not adjust the volume passing through it. A diverter would typically be used to redirect airflow rather than control its volume. A regulator is often associated with maintaining a specific pressure or flow rate but is not specifically used to adjust air volume in the same manner as a damper. Thus, a damper is the most appropriate device for altering the volume of air in a confined cross-section.