Final answer:
The sodium-potassium pump is an example of active transport, requiring energy to move ions against their concentration gradient.
Step-by-step explanation:
An example of active transport is C. Sodium-potassium pump in cell membranes, which moves sodium ions out of the cell and potassium ions into the cell. This process requires energy because it involves moving ions against their concentration gradient, from areas of lower concentration to areas of higher concentration. Unlike passive processes like diffusion or osmosis, which rely on the natural movement of substances from high to low concentration without the use of energy, active transport uses cellular energy (usually in the form of ATP) to move substances across the cell membrane.