Answer:
e. facilitated diffusion
Explanation:
Facilitated diffusion is one of the types of passive transport where the molecule moves from higher concentration to the lower concentration. Since the movement is along the concentration gradient, it does not require cellular energy.
It is called facilitated because some special proteins aid in the process. Many big and polar molecules can not cross the membrane easily despite the concentration gradient being present so they need the help of special proteins for movement. For example, water molecules are sometimes transported across the membrane using aquaporins.
