The two-wattmeter method is a technique used to measure the power in a three-phase circuit. It involves connecting two wattmeters to the circuit and using their readings to determine the total power.

In this method, the two wattmeters are connected such that one is connected to measure the power in one of the phases (W1) and the other is connected to measure the power in the sum of the other two phases (W2). The total power is then given by the sum of the readings of the two wattmeters.

In a balanced three-phase system, the power factor is defined as the cosine of the angle between the voltage and current waveforms. A power factor of 0.5 means that the voltage and current waveforms are out of phase by 60 degrees.

Let's analyze the readings of the two wattmeters in a balanced three-phase system with a power factor of 0.5:

1. The first wattmeter (W1) measures the power in one of the phases. Since the power factor is 0.5, the current waveform lags behind the voltage waveform by 60 degrees. As a result, the reading of W1 will be positive.

2. The second wattmeter (W2) measures the power in the sum of the other two phases. In a balanced system, the sum of the currents in the other two phases is zero. Therefore, the reading of W2 will be zero.

Since the total power is given by the sum of the readings of the two wattmeters, the total power in this case will be W1 + W2 = W1 + 0 = W1.

Therefore, in a balanced three-phase system with a power factor of 0.5, one of the wattmeters will read zero. This is because the sum of the currents in the other two phases is zero, resulting in zero power being measured by the second wattmeter.

Hence, the correct answer is option 'B' (Zero).