Which of the following describes the movement of ions across a membrane in response to a concentration gradient?

The correct choice describes passive diffusion as a process where ions move across a membrane along their concentration gradient, from an area of higher concentration to an area of lower concentration. This movement does not require energy input because it relies on the natural tendency of particles to spread out evenly in space, following the principles of thermodynamics.

In passive diffusion, ions will continue to move until there is an equal concentration on both sides of the membrane, achieving equilibrium. This is a fundamental concept in biology and is important for maintaining cellular homeostasis.

The other options involve different mechanisms of transport. Active transport requires energy to move ions against their concentration gradient. Facilitated transport also does not require energy but involves specific transport proteins to help move molecules that cannot easily cross the membrane. Secondary active transport utilizes the energy from the movement of other ions to move different ions against their gradients. Hence, these mechanisms differ from the process described in the question, which is purely about movement along a gradient without energy expenditure.