The correct answer is: A. in series with current coil.
A wattmeter is an instrument used to measure the power in an electric circuit. It consists of two coils, a current coil and a pressure coil. The current coil is connected in series with the load, and the pressure coil is connected in parallel with the load. The two coils are connected to a moving coil, which is deflected by the current flowing through the coils. The amount of deflection is proportional to the power in the circuit.
In a low power factor wattmeter, the compensating coil is connected in series with the current coil. This is done to compensate for the error caused by the inductance of the current coil. The inductance of the current coil causes a phase difference between the current and the voltage, which results in an error in the measurement of power. The compensating coil is designed to have an inductance that is equal and opposite to the inductance of the current coil. This cancels out the phase difference and results in an accurate measurement of power.
The other options are incorrect because they do not compensate for the error caused by the inductance of the current coil. Option B would result in a larger error, and options C and D would not result in any error.