Answer:
A) a single layer of flattened cells
Explanation:
When an epithelium is specialized for the passive diffusion of materials from one fluid to another, it must be a simple epithelium, that is, it has a single layer of cells, and these are flattened. Thus, the diffusion of different substances will be easier (and faster) since it will have to pass through less structures, such as fewer membranes. If it were a stratified epithelium, with many layers of stacked cells, the substance would take a long time and more energy would also be used for its transport. The same happens with the surface to be crossed, if they were large cube-shaped cells, transport would be more difficult and slow, however, being flattened cells these are very thin and there is little surface. An example of this is the endothelium of blood vessels, this is a simple flat epithelium that covers the vessels, so oxygen molecules can be easily exchanged with carbon dioxide in the pulmonary alveoli (which are also formed by flat cells ).
