Which of the following is a characteristic feature of active transport?

Active transport is characterized by its requirement for energy input. This process involves the movement of substances against their concentration gradient, meaning that molecules are transported from an area of lower concentration to an area of higher concentration. Such movement cannot occur spontaneously, as it defies the natural tendency of particles to spread out (diffuse) from areas of high concentration to low concentration.

In active transport, specific transport proteins, often referred to as pumps, utilize energy derived from molecules such as ATP to drive the process. This is crucial for maintaining cellular homeostasis and allowing cells to regulate their internal environment, for instance, by accumulating ions or nutrients that are present in lower concentrations outside the cell compared to the inside.

The other options describe processes that do not apply to active transport. While some transport mechanisms can occur passively or spontaneously without energy input, active transport specifically denotes a process that requires energy to operate efficiently against a concentration gradient.