Question

facilitated diffusion is most useful when... all of these. cells are dehydrated and need water. molecules are moving against the concentration gradient. molecules that do not fit through the cell membrane are moving passively.

Explanation:

Facilitated diffusion is a passive transport process (no energy required) that helps molecules (which can't pass through the cell membrane freely, e.g., due to size or charge) move across the membrane down their concentration gradient. Water moves via osmosis (a type of simple diffusion for water in many cases, not facilitated diffusion in typical scenarios of just needing water for dehydrated cells as simple diffusion can work for water in some membranes). Movement against the concentration gradient is active transport. But when molecules that can't pass through the membrane (due to structure) move passively (down their gradient), facilitated diffusion is used.

Answer:

molecules that do not fit through the cell membrane are moving passively.