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Energy stored in a capacitor over a cycle, when excited by an a.c. source is
The same as that due to a d.c. source of equivalent magnitude
Half of that due to a d.c. source of equivalent magnitude
Zero
None of the above
When a capacitor is excited by an AC source, the capacitor stores energy for half time period and delivers that energy in another time period. Hence total energy stored in a capacitor over a complete cycle, when excited by an AC source is zero.
This can be derived by plotting current and voltage and as current leads the voltage the product of current and voltage is positive for half cycle i.e. capacitor stores energy in half cycle and product is negative in other half cycle so capacitor delivers energy.
This can also be explained as, the final and initial value of voltage will be same after a complete cycle and since energy stored in a capacitor is 
. So, initial and final values of stored energy will also be same. Hence, energy stored will be zero.