The correct answer is: A. 2 to 3 times greater.
The cooling time constant is the time it takes for the motor to cool down to a certain temperature after it has been turned off. The heating time constant is the time it takes for the motor to heat up to a certain temperature after it has been turned on.
In self-cooled motors, the cooling time constant is about 2 to 3 times greater than the heating time constant because cooling conditions are worse at standstill. This is because when the motor is running, the air flow around the motor helps to cool it down. However, when the motor is stopped, there is no air flow and the motor cools down more slowly.
Option B is incorrect because the cooling time constant is not 3 to 4 times greater than the heating time constant. Option C is incorrect because the cooling time constant is not 4 to 5 times greater than the heating time constant. Option D is incorrect because the cooling time constant is not none of the above.