In which case would facilitated diffusion be needed? A. With large polar molecules moving against the concentration
gradient
B. With very small non polar particles moving down the
concentration gradient.
C. With very small polar particles moving against the
concentration gradient.
D. With large non polar molecules moving down the concentration
gradient.

The case in which facilitated diffusion would be needed is in the transport of large non-polar molecules moving down the concentration gradient.

Explanation:

Facilitated diffusion (FD) involves a form of passive transport, that is, it does not require energy and mobilizes substances across the cell membrane in favor of a concentration gradient.

The passive transport mechanism of FD is necessary only in the case of transport of very large molecules or polar molecules that:

For FD to occur, it requires the use of a protein, which acts as a carrier of the molecules to be mobilized.

The other options are not correct because:

    A and C. Passive transport, as facilited diffusion,  implies movement in favor of a concentration gradient.

    B and C. Non-polar small or very small molecules can cross the membrane by simple diffusion.

i think it’s c but i might be wrong. if i am let me know! have a good rest of your day as well!