The correct answer is: D. Supply voltage = back e.m.f.
The back emf of a DC motor is the voltage that is generated by the motor itself. It opposes the applied voltage, and the difference between the two is the voltage that drives the current through the motor. The power in a DC motor is given by the equation $P = I^2R$, where $I$ is the current and $R$ is the resistance of the motor.
When the back emf is equal to the applied voltage, the current through the motor is zero. This means that there is no power loss in the motor, and the maximum power is transferred from the power source to the motor.
The other options are incorrect because they do not result in zero current through the motor. Option A would result in a current of 2 times the rated current, which would cause the motor to overheat. Option B would result in a current of half the rated current, which would not provide enough power to the motor. Option C would result in a current that is undefined, as the back emf and the applied voltage are in opposite directions.