Final answer:
At the functional residual capacity, the correct statement describes that the lungs and the chest wall tend to collapse due to their respective elastic properties, which means 'Both a and c are true' (d) is the correct answer choice.
Step-by-step explanation:
Functional residual capacity (FRC) is the condition in the lung that is present at the end of passive expiration. This is the amount of air that remains in the lungs after a normal, passive exhalation. At this point, the lungs tend to collapse due to their elastic properties. This is because the elastic recoil of the lungs is balanced by the opposing force of the chest wall seeking to expand outward, both forces equilibrate at the FRC. The intrapleural pressure is negative relative to both atmospheric pressure and intra-alveolar pressure to maintain the lungs in an inflated state, and the chest wall tends to collapse due to its elastic properties. However, the intra-alveolar pressure will equalize with the atmospheric pressure at rest. Therefore, the correct answer to the student's question is that at the functional residual capacity, 'The lungs tend to collapse due to their elastic properties' (b), and 'The chest wall tends to collapse due to its elastic properties' (c), making 'Both a and c are true' (d) the correct choice. The atmospheric, intra-alveolar, and intra-pleural pressures are not all equal (a), so this part of the statement is not accurate in this context.