The correct answer is B) voltage of the incoming machine is at or slightly above the bus voltage. When paralleling two DC generators, it is crucial that the voltage of the incoming machine is at or slightly above the bus voltage. This ensures that power flows from the incoming generator to the bus, rather than in the opposite direction, which could potentially damage the generators. The voltage must be closely matched to prevent circulating currents between the generators. The other options are incorrect because: A) matching the current is not required, C) a synchroscope is used for paralleling AC generators, not DC generators, and D) frequency is not a factor when paralleling DC generators, as they operate at a constant speed.