Which of the following loss in a D.C. generator is dissipated in the form of heat?

The correct answer is D. All of the above.

Mechanical loss is the loss of energy due to friction and windage. Core loss is the loss of energy due to eddy currents and hysteresis in the magnetic core. Copper loss is the loss of energy due to the resistance of the windings. All of these losses are dissipated in the form of heat.

Mechanical loss is caused by the friction between the moving parts of the generator, such as the rotor and the stator. Windage loss is caused by the air resistance to the motion of the rotor. Core loss is caused by the eddy currents and hysteresis that are induced in the magnetic core when the generator is energized. Copper loss is caused by the resistance of the windings.

All of these losses are dissipated in the form of heat. The heat generated by these losses must be removed from the generator to prevent it from overheating. This is done by using a cooling system, such as a fan or a water-cooling system.