The correct answer is: C. Motor is run at reduced speed.
Rheostatic braking is a type of braking in which the resistance of a rheostat is used to dissipate the energy of a moving object. In the case of a DC series motor, the rheostat is placed in series with the motor. When the motor is turned off, the current through the rheostat causes it to heat up. This heat energy is then used to slow down the motor.
Option A is incorrect because the motor is not run as a generator during rheostatic braking. A generator converts mechanical energy into electrical energy, while a motor converts electrical energy into mechanical energy. During rheostatic braking, the motor is still converting electrical energy into mechanical energy, but the energy is being dissipated in the rheostat instead of being used to turn a load.
Option B is incorrect because the motor is not reversed in direction during rheostatic braking. The direction of rotation of a DC series motor is determined by the polarity of the applied voltage. When the motor is turned off, the polarity of the applied voltage does not change, so the direction of rotation does not change either.
Option D is incorrect because rheostatic braking is a real and effective way to slow down a DC series motor.