The power factor of a single phase load can be calculated if the instruments available are

The correct answer is: A. one voltmeter and one ammeter.

To calculate the power factor of a single phase load, you need to know the voltage and current. You can measure these with a voltmeter and an ammeter. The power factor is then calculated as:

Power factor = cos(phi) = V/I

where V is the voltage and I is the current.

Option B is incorrect because you do not need a wattmeter to calculate the power factor. Option C is incorrect because you do not need an energy meter to calculate the power factor. Option D is incorrect because only option A is correct.