Which of the following describes an active device?

An active device is characterized by its need for an external power source to function properly. In this context, the correct answer identifies that an active device requires voltage to operate. This distinguishes it from passive devices, which can rely solely on the energy provided by the signals they process without needing an independent power source.

Active devices include components such as transistors and operational amplifiers that actively control the flow of electricity and can amplify signals. The requirement for voltage means these devices can manipulate signals in a way that passive devices, which merely harness and redistribute energy without additional power input, cannot.

The other options describe characteristics of passive devices or imply functionality that does not align with the definition of active devices. Active devices are not defined by their ability to store energy, and they definitely contrast with any device that functions independently of an external power source. Additionally, active devices do not passively regulate voltage; rather, they actively control and process electrical signals to achieve desired behaviors, making the focus on their voltage requirements a key distinguishing factor.