The correct answer is A. Half wave rectified sine wave.
A half wave rectified sine wave is a type of waveform that is created by taking the positive half-cycles of a sine wave and discarding the negative half-cycles. This results in a waveform that has a positive peak value and a zero average value.
A triangular wave is a type of waveform that has a constant slope. This means that the average value of a triangular wave is equal to its peak value divided by two.
A sine wave is a type of waveform that has a sinusoidal shape. This means that the average value of a sine wave is equal to its peak value divided by $\pi$.
A square wave is a type of waveform that has a constant value for half of its period and a zero value for the other half of its period. This means that the average value of a square wave is equal to its peak value divided by two.
Therefore, the half wave rectified sine wave has the least mean value because it has a zero average value.