Final answer:
The pressure in the alveoli is greater than in the pleural cavity due to the elastic recoil of the lungs, causing the lungs to contract after expansion and create a pressure gradient that expels air.
Step-by-step explanation:
The pressure in the alveoli is greater than in the pleural cavity, mainly due to the elastic recoil of the lungs and the negative intrapleural pressure. During exhalation, the lung tissue's natural elasticity causes the lungs to recoil, increasing the intrapulmonary pressure and creating a gradient that forces air out of the alveoli. On the other hand, intrapleural pressure remains negative to keep the lungs attached to the chest wall. The presence of surfactant reduces surface tension in the alveoli, yet it does not make the alveolar pressure greater than pleural pressure, but rather helps the alveoli to expand during inhalation. Therefore, the correct answer to the student's question would be (b) Elastic recoil of lungs.