Facilitated diffusion is best described as:

Facilitated diffusion is best characterized as a process that occurs through membrane channels. This type of transport mechanism allows specific molecules or ions to cross the cell membrane more easily than they could through the lipid bilayer. In facilitated diffusion, transport proteins are embedded in the membrane and provide passageways for these substances, which are often polar or larger molecules that cannot diffuse freely through the lipid membrane. These transport proteins can be either carrier proteins that change shape to move the molecules across the membrane or channel proteins that form pores.

Facilitated diffusion is a passive process, meaning it does not require energy expenditure, and it occurs down the concentration gradient—moving substances from areas of higher concentration to areas of lower concentration until equilibrium is reached. This distinguishes facilitated diffusion from active transport, which requires energy to move substances against their concentration gradient. Therefore, the option correctly highlights the nature of facilitated diffusion as involving specific membrane channels that assist in the transport of particular molecules.