What type of current flows in one direction only?

Direct current (DC) is characterized by the flow of electric charge in a single direction. This type of current is typically produced by sources such as batteries and solar cells, where the charge flows steadily and does not change direction.

In contrast, alternating current (AC) reverses direction periodically, which is the primary difference between the two types of current. High frequency and low frequency currents refer to variations in the rate at which the AC changes direction, but they still involve bidirectional flow rather than the unidirectional flow of DC. Hence, direct current uniquely represents a constant, linear flow of electricity in one direction, making it the correct answer to the question.