In a single-phase half-wave rectifier, the input AC voltage is converted into a pulsating DC voltage. This rectifier circuit uses a diode that conducts current only during the positive half-cycle of the input AC signal. As a result, the output current is present only during the positive half-cycle of the input waveform.

Average Current in a Single-Phase Half-Wave Rectifier:
The average current is the average value of the rectified output current over a complete cycle of the input waveform. In a single-phase half-wave rectifier, the average current can be calculated as the ratio of the area under the positive half-cycle of the rectified waveform to the time period of that half-cycle.

Explanation of the Correct Answer (Option D):
The correct answer is option D, which states that the average current in a single-phase half-wave rectifier is always negative. This answer can be explained as follows:

1. Operation of the Half-Wave Rectifier:
During the positive half-cycle of the input AC signal, the diode in the half-wave rectifier conducts and allows current to flow through the load resistance. This results in a positive half-cycle of the output waveform. However, during the negative half-cycle of the input AC signal, the diode blocks the current flow and there is no output current.

2. Average Current Calculation:
To calculate the average current, we consider the area under the positive half-cycle of the rectified waveform. Since the output current is present only during the positive half-cycle, the area under this portion represents the contribution to the average current. As the waveform is entirely above the time-axis, the area under it is negative.

3. Negative Average Current:
Due to the rectification process in the half-wave rectifier, the output current flows only during the positive half-cycle of the input waveform. As a result, the average current is negative because the area under the positive half-cycle of the rectified waveform contributes to the average current, and this area is negative.

Conclusion:
In a single-phase half-wave rectifier, the average current over one full cycle is always negative. This is because the rectification process allows current flow only during the positive half-cycle of the input waveform, resulting in a negative contribution to the average current.