Final Answer:
The movement of an ion down its concentration gradient is called A. passive transport.
Step-by-step explanation:
Passive transport refers to the movement of ions or molecules across a cell membrane without the expenditure of energy. When an ion moves down its concentration gradient, it means it is moving from an area of higher concentration to an area of lower concentration. This can occur through various passive transport mechanisms such as simple diffusion, facilitated diffusion, or through ion channels. Passive transport is driven by the inherent kinetic energy of particles, and it does not require the direct input of metabolic energy. On the other hand, active transport (Option C) involves the movement of ions or molecules against their concentration gradient, requiring energy input, often from ATP. Osmosis (Option D) specifically refers to the passive movement of water molecules across a semi-permeable membrane.
Option A. passive transport is the answer.