Carrier-mediated transport of a substance down its concentration gradient is called _______________.endocytosisactive transportfacilitated diffusionexocytosis

The correct answer to the question is facilitated diffusion.

Explanation:

Facilitated diffusion is a process in which substances move across a cell membrane through protein channels or carriers, relying on the concentration gradient without the expenditure of energy. This process is specifically designed for substances that cannot freely pass through the lipid bilayer of the cell membrane. The other options listed are not correct in this context:

1. Endocytosis refers to the process of internalizing substances into the cell.
2. Active transport involves moving substances against their concentration gradient, which requires energy.
3. Exocytosis is the process of expelling substances from the cell to the external environment.

Thus, facilitated diffusion is specifically concerned with the transport of substances down their concentration gradient, making it the correct choice.