The correct answer is C) temperature of the compartment where the motor is located. Ambient temperature refers to the temperature of the surrounding environment or compartment where the motor is located. This is the temperature that the motor must operate in, regardless of the amount of heat the motor generates itself. The ambient temperature is an important factor in determining the proper motor rating and ventilation requirements, as the motor must be able to operate safely and efficiently in the given environment. The other answer choices are incorrect because they do not accurately define ambient temperature. A) and D) refer to the temperature rise or load conditions of the motor itself, not the surrounding environment. B) refers to a relative temperature difference, not the actual ambient temperature.