Final answer:
The correct answer to the question is 'The respiratory membrane consists of an endothelial cell of a capillary and the plasma membrane of an alveolar type I cell.' This membrane plays a critical role in gas exchange within the lungs.
Step-by-step explanation:
The respiratory membrane consists of an endothelial cell of a capillary and the plasma membrane of an alveolar type I cell. The simple squamous epithelium formed by type I alveolar cells is extremely thin and is part of the blood-air barrier which facilitates simple diffusion of gases. It allows oxygen to enter the blood and carbon dioxide to be expelled into the alveolar air. The type II alveolar cells are also present in the alveolar wall but are responsible for secreting pulmonary surfactant to reduce surface tension, not for the formation of the respiratory membrane itself.