The correct answer is D. Rise in temperature of ventilating air.
Iron losses are the losses that occur in the magnetic core of a DC generator. These losses are caused by the eddy currents and hysteresis losses that occur in the core. Eddy currents are caused by the changing magnetic field in the core. Hysteresis losses are caused by the reversal of the magnetic field in the core.
Iron losses cause the core to heat up. This heating can cause the generator to become less efficient. It can also cause the generator to overheat and damage the insulation.
The rise in temperature of the ventilating air is not caused by iron losses. The ventilating air is used to cool the generator. The rise in temperature of the ventilating air is caused by the heat generated by the generator.
The other options are all caused by iron losses.
A. Loss of efficiency: Iron losses cause the generator to become less efficient. This is because the iron losses cause the core to heat up. The heating of the core causes the resistance of the core to increase. The increase in resistance causes the generator to become less efficient.
B. Excessive heating of core: Iron losses cause the core to heat up. This heating can cause the generator to become less efficient. It can also cause the generator to overheat and damage the insulation.
C. Increase in terminal voltage: Iron losses do not cause the terminal voltage of the generator to increase. The terminal voltage of the generator is determined by the speed of the generator and the field current.
I hope this helps!