The correct answer is A) temperature of the evaporator coil outlet. The thermal expansion valve reacts directly to changes in the temperature of the refrigerant leaving the evaporator coil. This is because the primary function of the thermal expansion valve is to regulate the flow of liquid refrigerant into the evaporator coil based on the temperature of the refrigerant at the coil outlet. This ensures the evaporator is fully utilized without liquid refrigerant returning to the compressor. The other options are incorrect because they do not directly influence the operation of the thermal expansion valve. The temperature of the space being cooled (B), the liquid refrigerant pressure at the solenoid valve (C), and the pressure drop across the evaporator coils (D) are not the primary parameters the thermal expansion valve reacts to.