Which of the following serves as a passive process in the transport of substances across membranes?

Facilitated diffusion is indeed a passive process that plays a critical role in the transport of substances across cell membranes. It involves the movement of molecules from a region of higher concentration to a region of lower concentration, thereby following the natural concentration gradient. This process utilizes specific carrier proteins or channels embedded in the cell membrane to help transport substances that cannot easily pass through the lipid bilayer due to their size, charge, or polarity.

Since facilitated diffusion does not require energy expenditure from the cell, it qualifies as a passive form of transport. This is in contrast to active transport, which requires energy to move substances against their concentration gradient. Endocytosis and phagocytosis are also active processes, as they involve the engulfing of substances into the cell, requiring energy to change the cell membrane's shape. Consequently, facilitated diffusion is distinguished by its reliance on concentration gradients and lack of energy requirement, making it a quintessential example of passive transport across membranes.