Final answer:
The correct answer is (c), which states that during exhalation, the diaphragm relaxes, increasing intrapulmonic pressure and allowing air to be expelled from the lungs in a passive process.
Step-by-step explanation:
During exhalation, several changes occur in the thoracic cavity and the lungs. As specified by the passages provided, the diaphragm relaxes and moves upward, which decreases the volume of the thorax. This movement leads to an increase in intrapulmonic pressure, making it higher than the atmospheric pressure outside the lungs. Consequently, air is expelled from the lungs in a passive process, driven by the elasticity of the lung tissues and the relaxation of the muscles. Thus, the correct answer to the question is option (c): the diaphragm relaxes, increasing intrapulmonic pressure.
Option (d) suggests that accessory muscles may contract, pulling the ribcage upward during exhalation. However, this is not typically the case during gentle exhalation, as it is a passive process. Accessory muscles are more likely to be involved during forced exhalation.