The correct answer is: D. zero, negative and positive sequence currents.
A balanced three-phase system consists of three equal-magnitude, sinusoidal currents that are 120 degrees apart in phase. The currents can be represented as a phasor diagram, with the positive sequence current at the top, the negative sequence current at the bottom, and the zero sequence current in the middle.
The zero sequence current is the current that is common to all three phases. It is zero in magnitude and has a phase angle of zero. The positive sequence current is the current that is in phase with the voltage. The negative sequence current is the current that is 120 degrees out of phase with the voltage.
In a balanced three-phase system, the sum of the three currents is zero. This is because the positive sequence current is equal to the negative sequence current, but with a 120 degree phase shift. The zero sequence current is also equal to the positive and negative sequence currents, but with a 180 degree phase shift.
The zero sequence current is often neglected in three-phase power systems, as it does not contribute to the power flow. However, it can be important in some cases, such as when there is a fault on a line.