Final answer:
Facilitated diffusion is dependent on the diffusion rate of the carrier protein, as these proteins help transport molecules across the cell membrane in a saturable and selective manner.
Step-by-step explanation:
Facilitated diffusion is dependent upon the diffusion rate of the carrier protein. This process involves the movement of molecules across the cell membrane through specific proteins without the expenditure of cellular energy. The molecules move from an area of higher concentration to one of lower concentration, with the carrier proteins determining the rate at which the diffusion occurs. Unlike simple diffusion, facilitated diffusion is saturable; meaning that there's a maximum rate at which the carrier proteins can work. Once all the binding sites of the carrier proteins are occupied, the rate will not increase further, even if the concentration of the diffusing molecule increases. Therefore, the correct answer to the question is B. Diffusion rate of the carrier protein. Carrier proteins are selective and can transport molecules like glucose and certain ions across membranes at different rates, thus being a critical factor in facilitated diffusion.