Facilitated diffusion is a form of passive transport utilizing membrane proteins. when is facilitated diffusion used rather than diffusion?

Explanation:

Facilitated diffusion is used for the transport of large molecules unable to cross the plasma membrane at will. However, passive diffusion, describes the movement of substances (small molecules and ions) across the membrane, along their concentration gradient.

Further Explanation:

Transmembrane proteins are embedded within the membrane from the extracellular fluid to the cytoplasm, and are sometimes attached to glycoproteins (proteins attached to carbohydrates) which function as cell surface markers.

Carrier proteins and channel proteins are the two major classes of membrane transport proteins. Carrier proteins (also called carriers, permeases, or transporters) bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane

Transport proteins spanning the plasma membrane facilitate the movement of ions and other complex, polar molecules  which are typically prevented from moving across the membrane.

There are two types:

Carrier proteins bind specifically bind to molecules and move them down concentration gradients.Along with using facilitated diffusion, carrier proteins also use energy in the form of  ATP and modifiy solute-specific regions, that aid in regulating ion exchange, through the hydrophobic layer of the plasma membrane.

Channel proteins which are pores filled with water versus enabling charged molecules to diffuse across the membrane,  from regions of high concentration to regions of lower concentration. This is a passive part of facilitated diffusion.

Explanation:

the answer to your question is b