The correct answer is: C. three-phase winding
A phase shifting transformer is a type of transformer that is used to shift the phase angle of an alternating current (AC) signal. It has three windings, which are connected to the three phases of an AC power supply. The windings are arranged in such a way that the output voltage of the transformer is out of phase with the input voltage.
A single-phase winding would not be able to shift the phase angle of an AC signal, as it would only produce a single output voltage that is in phase with the input voltage. A two-phase winding could be used to shift the phase angle of an AC signal, but it would be less efficient than a three-phase winding. This is because a two-phase winding has two output voltages that are 90 degrees out of phase with each other, while a three-phase winding has three output voltages that are 120 degrees out of phase with each other.
Therefore, the stator of a phase shifting transformer for use in conjunction with an A.C. potentiometer usually has a three-phase winding.