The correct answer is: A. unity.
In a 3-phase power measurement by two wattmeter method, the power factor of the load is unity if both the wattmeters have identical readings. This is because the two wattmeters measure the power in the two phases of the load, and if the power factor is unity, then the power in each phase is equal.
Option B is incorrect because a power factor of 0.8 lagging indicates that the load is consuming reactive power, which means that the current is lagging the voltage. This would cause the two wattmeters to have different readings.
Option C is incorrect because a power factor of 0.8 leading indicates that the load is generating reactive power, which means that the current is leading the voltage. This would also cause the two wattmeters to have different readings.
Option D is incorrect because a power factor of zero indicates that the load is not consuming any power. This would mean that the two wattmeters would have zero readings.