Final answer:
A damper is a device that modulates the volume of air by adjusting the cross section area through which the air passes, thereby affecting airflow in ventilation systems (option A).
Step-by-step explanation:
A damper is a device used to alter the volume of air passing through a confined cross section by changing the size of the cross section area. This mechanical device can be found in ventilation systems, where it regulates airflow and thus controls the temperature and air quality within a space. By adjusting the damper, the size of the cross section through which air passes can be increased or decreased, which in turn affects the volume of air flow.
Hence, the answer is option A.