Concept:Active transport is the movement of substances against a concentration gradient, using metabolic energy in the form of ATP.
Explanation:Oxygen and carbon dioxide move by simple diffusion because they are small, uncharged molecules.
Water moves mainly by osmosis, which is a passive process.
Sodium ions have a positive charge, so they cannot easily pass through the lipid bilayer of the cell membrane.
In the kidney, sodium is reabsorbed from the filtrate into the blood against a concentration gradient.
This process requires energy and carrier proteins, making it a clear example of active transport.
Answer:D. Sodium being reabsorbed in the kidney.