If the load is not balanced on a three-phase, four-wire power system, this means that,
a. The current in the fourth wire (neutral) will not be zero
In a three-phase, four-wire power system, the load is said to be balanced when the currents in the three phases are equal and evenly spaced apart in terms of phase angle. In a balanced system, the currents in the three phases add up to zero, resulting in no current flowing through the neutral wire.
If the load is not balanced, meaning the currents in the three phases are not equal or not evenly spaced, the currents will not add up to zero, and there will be a non-zero current flowing through the neutral wire. This is the reason why option "a" is correct: the current in the fourth wire (neutral) will not be zero if the load is not balanced.