The correct answer is A) direct current. Direct current (DC) is the correct term for current that flows in only one direction, as opposed to alternating current (AC) which changes direction periodically. This is a fundamental principle in electrical engineering and is an important concept tested on the US Coast Guard Captain's License Examinations. The other answer choices are incorrect - alternating current (AC) refers to current that reverses direction, sinusoidal current is a specific type of AC, and omnidirectional current does not have a clear meaning in this context.