Which of the following is a characteristic of direct current (DC)?

Direct current (DC) is defined by its ability to flow continuously in one direction. This consistent flow allows it to provide a steady voltage and current, which is crucial for many electronic devices and batteries. Unlike alternating current (AC), where the current changes its direction periodically, DC maintains a constant polarity and does not reverse. This characteristic makes DC ideal for powering devices that require a constant voltage, such as computers, battery-operated devices, and various electronic circuits.

In contrast, other options relate to characteristics not associated with DC. The notion of changing direction periodically, for instance, applies to AC rather than DC. Similarly, saying that DC has variable frequency is misleading, as DC is characterized by a frequency of zero – it doesn't oscillate or alternate like AC does. Lastly, DC is not primarily used for AC appliances; those appliances are specifically designed to operate on alternating current. Therefore, the attribute of flowing continuously in one direction distinctly identifies direct current from other types of electrical current.