The correct answer is: D. The voltage reverses polarity periodically.
A pulsating DC voltage is a type of voltage that has a constant average value but varies periodically in amplitude. This means that the voltage has a positive and negative peak, and the polarity of the voltage reverses between these peaks.
Option A is incorrect because a pulsating DC voltage does not remain at one polarity and at a steady level. The voltage changes value periodically.
Option B is incorrect because a pulsating DC voltage does not remain at one polarity but changes value periodically. The voltage reverses polarity periodically.
Option C is incorrect because a pulsating DC voltage does not cause current to flow in two directions at the same time. The current only flows in one direction, but the polarity of the voltage reverses periodically.
Option E is incorrect because the statement in option D is the most accurate description of the nature of a pulsating DC voltage.