What process involves the movement of a substance from an area of high concentration to an area of low concentration?

The correct choice, diffusion, describes the natural tendency of particles to move from an area where they are in higher concentration to an area of lower concentration. This process occurs spontaneously and is driven by the kinetic energy of the molecules, allowing them to spread out evenly in a given space.

In biological systems, diffusion is crucial for many physiological processes, such as the movement of oxygen and carbon dioxide across cell membranes, where it ensures the proper exchange of gases necessary for cellular respiration. The simplicity and efficiency of diffusion make it an essential mechanism for maintaining homeostasis within cells and organisms.

While facilitated diffusion and osmosis are specific types of diffusion, they involve additional factors such as carrier proteins and the movement of water, respectively. Active transport, on the other hand, is a process that requires energy to move substances against their concentration gradient, which is fundamentally different from the passive nature of diffusion.