The correct answer is B) when the evaporator has just begun feeding at relatively high box temperature. This is because the thermostatic expansion valve operates based on the temperature and pressure in the evaporator. When the box temperature is relatively high, the refrigerant flow through the valve will be greatest to rapidly cool the evaporator and bring the box temperature down. As the box temperature drops, the flow through the valve will decrease to maintain the proper superheat in the evaporator. The other options are incorrect because: A) equal low and high side pressures would indicate a system imbalance, not optimal operation; C) equal low side and bulb pressures would indicate the valve is not responding to changes in evaporator conditions; and D) just before the evaporator stops feeding would indicate the valve is not providing enough flow to maintain the proper temperature.