Which of the following best describes a servo?

A servo is a device widely recognized for its capability to control a specific physical quantity, such as position, velocity, or acceleration. Servos employ a feedback mechanism that allows them to receive input signals and adjust their output to maintain the desired state. This functionality is crucial in applications like robotics, automation, and remote-control devices, where precise control over movement is required.

The other options do not accurately describe the core function of a servo. While a device that generates electrical power or steps up voltage might serve specific purposes in electrical systems, they do not encompass the control aspects unique to servos. Additionally, a device that operates at fixed speed lacks the dynamic adjustment characteristic of servos. Thus, the defining feature that sets servos apart is their ability to actively control and adjust physical quantities, making the choice that identifies this function the most suitable.