Which of the following is not a characteristic of DC circuits?

The choice of variable frequency as the characteristic that is not associated with DC circuits is accurate because direct current (DC) is defined by a constant voltage that does not change over time, leading to a steady flow of electric charge. Essentially, in DC circuits, the voltage remains constant, and there is no frequency involved, as frequency pertains to alternating current (AC) where the current periodically reverses direction and varies in amplitude.

Constant current flow is a fundamental characteristic of DC circuits since they maintain a uniform flow of electric charge. Stable voltage levels are also inherent to DC circuits, which ideally have a fixed voltage throughout the circuit operation. Directionally fixed electrons further describe the nature of DC, as the flow is consistently in one direction. Thus, identifying variable frequency as not applicable to DC circuits highlights the distinct difference between DC and AC.