Which component would be used if heat pump equipment begins the defrost mode of operation by temperature?

The correct choice is to use a thermistor in this scenario because a thermistor is a temperature-sensitive resistor with a high level of precision in sensing temperature changes. It is commonly used in heat pump systems for monitoring the low-temperature conditions that prompt the unit to switch into defrost mode.

When the outdoor temperature drops to a certain threshold, the thermistor can detect this and signal the heat pump to enter the defrost mode to prevent frost buildup on the outdoor coil. Thermistors react quickly and provide accurate readings, making them ideal for such applications where thermal conditions need constant monitoring.

Other components like pressure switches might be involved in monitoring refrigerant pressures but do not directly sense temperature. Thermostats serve a broader function in controlling heating or cooling in response to general air temperature, and heat sensors are less specific in their application related to system operation and may not provide the precise data needed to initiate defrost operations based solely on temperature readings.