The correct answer is D. zero.
In a 3-phase power measurement by two wattmeter method, the reading of one of the wattmeter was zero. This means that the power consumed by the load is zero. The power factor of a load is the ratio of the real power to the apparent power. The real power is the power that is used to do work, while the apparent power is the product of the voltage and current. The power factor is given by the following equation:
$Power factor = \frac{Real power}{Apparent power}$
If the real power is zero, then the power factor is also zero. This is because the apparent power is always greater than or equal to the real power.
The other options are incorrect because they are all values of power factor that are greater than zero.