The correct answer is: C. low internal resistance.
A constant-voltage generator is a type of generator that maintains a constant output voltage regardless of the load. This is achieved by having a low internal resistance, which means that the generator can supply a large amount of current without dropping the output voltage significantly.
A generator with a high internal resistance would have a lower output voltage under load. This is because the internal resistance would cause a voltage drop across it, which would reduce the amount of voltage that is available to the load.
A generator with a minimum efficiency would not be able to produce as much power as a generator with a higher efficiency. This is because some of the power generated by the generator would be lost as heat due to the internal resistance.
A generator with a minimum current capacity would not be able to supply as much current to a load as a generator with a higher current capacity. This is because the internal resistance would limit the amount of current that can be supplied.