Question

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

Explanation:

Facilitated diffusion is a type of passive transport. It helps molecules that cannot pass through the cell membrane (due to size, charge, etc.) move across the membrane from an area of higher concentration to an area of lower concentration (down the concentration gradient).

• Molecules moving against the concentration gradient is active transport, not facilitated diffusion.
• "All of these" is incorrect as the first option is wrong.
• When cells are dehydrated and need water, simple diffusion (osmosis for water) is mainly involved (though aquaporins can be involved in some water transport which is a form of facilitated diffusion for water in some cases, but the key feature of facilitated diffusion is for molecules that can't pass through the membrane easily). The most accurate description related to the core concept of facilitated diffusion is for molecules that do not fit through the cell membrane moving passively.

Answer:

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