Which of the following describes passive membrane transport processes?

Passive membrane transport processes are characterized by the movement of substances across a cell membrane without the need for energy input from ATP. This process relies on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration, which is referred to as moving down the concentration gradient.

In passive transport, substances such as oxygen, carbon dioxide, and certain ions can permeate the membrane freely or with the assistance of specific proteins (facilitated diffusion) that do not require energy expenditure. The fundamental principle is that molecules naturally move to achieve equilibrium, and this is why option C accurately describes passive membrane transport.