Which characteristic is true for direct current (DC)?

The characteristic that is true for direct current (DC) is that it has a current polarity. In DC, the flow of electric charge is constant and unidirectional, meaning that the current flows in one direction only. This results in a fixed polarity, where the positive and negative terminals remain the same throughout the operation. This unidirectionality is what differentiates DC from alternating current (AC), which periodically changes direction.

The other options either describe AC characteristics or are incorrect in relation to DC. For example, the statement about periodically reversing direction describes how AC works, while saying it flows in alternating cycles also pertains to AC. The mention of "used primarily in AC devices" misrepresents the application of DC, as DC is typically used in batteries, electronic devices, and circuits requiring steady voltage. Thus, the defining characteristic of DC having a current polarity underscores its unique behavior in electrical systems.