The correct answer is: B. balance loads on both sides of the neutral.
In a D.C. 3-wire distribution system, balancer fields are cross-connected in order to balance loads on both sides of the neutral. This is done by connecting the two balancer fields in opposite directions. This creates a magnetic field that opposes the current in the load, which helps to balance the load on both sides of the neutral.
Option A is incorrect because balancer fields are not used to boost the generated voltage. Option C is incorrect because balancer fields are not used to make both machines run as unloaded motors. Option D is incorrect because balancer fields are not used to equalize voltages on the positive and negative outers.