Gaseous exchange in Aves occurs in the

Gaseous exchange in Aves (birds) occurs in the air sacs. These air sacs are connected to the lungs and extend throughout the bird's body, including into the hollow bones. As the bird breathes, air moves through the trachea and into the posterior air sacs, then into the lungs, and finally into the anterior air sacs before being expelled out of the body. During this process, the air sacs act as bellows, keeping air constantly flowing over the respiratory surfaces of the lungs, ensuring efficient gas exchange. Therefore, the correct answer is option C - air sacs.