Final answer:
Tidal volume (TV) is the amount of air that is inspired and expired during normal breathing, typically around 500 milliliters for an average adult. Therefore, the correct option is a.
Step-by-step explanation:
The amount of air breathed in and out during normal respiration is called tidal volume (TV). This is the amount of air that is inspired and expired during normal breathing, which is about 500 milliliters for an average adult. This volume is distinct from other volumes such as total lung capacity (TLC), which is the sum of the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.