In the case of an unsymmetrical alternating current the average value must always be taken over

The correct answer is D. The whole cycle.

The average value of an alternating current is the total area under the curve divided by the time period. In the case of an unsymmetrical alternating current, the area under the curve will be different for the positive and negative half-cycles. Therefore, the average value must be taken over the whole cycle in order to get an accurate result.

Option A is incorrect because the unsymmetrical part of the wave form may not represent the entire cycle.

Option B is incorrect because the quarter cycle may not represent the entire cycle.

Option C is incorrect because the half cycle may not represent the entire cycle.