If current varies periodically from zero to a maximum, back to zero, and then repeats, the signal is:

ANS:C - pulsating

A pure direct current is a signal that has a constant value. It may be positive or negative but not both.

A pulsating direct current is a result from rectification of AC to DC since the alternations are not completely smoothen out and there is a need for further filtration and regulation to keep it on a constant level.