Facilitated diffusion occurs:
into the cell only.
out of the cell only.
in either direction depending on the temperature.
in either direction depending on the concentration gradient of the molecule.
in either direction depending on the size of the molecule.

in either direction depending on the concentration gradient of the molecule.

Explanation:a cell is constantly in contact with it's external environment.This means that substances are constantly going in and out of the cell.

One of the ways in which substance enter and leave a cell is facilitated diffusion.Facilitated diffusion transports molecules ,which normally cannot move across the cell membrane.These molecules include glucose,amino acids,ions etc.

In facilitated diffusion,carrier molecules (proteins) which are embedded within the cell membrane, are needed to convey these solutes.

Movement across the cell membrane occurs due to concentration gradient,that is,the difference in the concentration of solute inside and outside of the cell. As a result, substances move from a region where the concentration is high to a region where the concentration is low. Thus the movement is concentration dependent.

the answer is:

carbon dioxide