The correct answer is: A. varying voltage produces the charging and discharging currents.
A capacitor is a device that stores electrical energy in an electric field. When an alternating voltage is applied to a capacitor, the voltage across the capacitor changes with time. This causes the current through the capacitor to also change with time. The current through a capacitor is proportional to the rate of change of the voltage across it. So, when an alternating voltage is applied to a capacitor, the current through the capacitor will be alternating current.
Option B is incorrect because the peak value of the voltage does not affect the current through the capacitor. The current through a capacitor is proportional to the rate of change of the voltage across it, not the peak value of the voltage.
Option C is incorrect because the charging current can only flow when the capacitor is being charged. The charging current is the current that flows into the capacitor when it is being charged. The discharging current is the current that flows out of the capacitor when it is being discharged.
Option D is incorrect because the discharge current can only flow when the capacitor is being discharged. The discharge current is the current that flows out of the capacitor when it is being discharged.