Which of the following correctly defines direct current (DC)?

Direct current (DC) is defined as an electrical current that flows in one direction only. This characteristic distinguishes it from alternating current (AC), which periodically reverses its direction. DC is commonly used in various applications, such as batteries, electronic devices, and some types of motors, where a constant voltage is necessary for proper operation. The one-directional flow results in a stable voltage level over time, making it ideal for many electronic circuits and systems that require consistent operation without fluctuations.

In contrast, the other options describe different types of electrical currents or misleading concepts. One option speaks to the behavior of alternating current, which changes direction. Another option refers to variations in amplitude, typical of AC where the voltage level fluctuates. Lastly, the generation of DC from an AC source pertains more to conversion methods rather than defining what direct current itself is. Thus, the specific and accurate definition of DC is a current that flows in one direction only, confirming that option B is the correct choice.