Answer:
B. Don't let the energy into the fridge.
Step-by-step explanation:
Temperature is actually "heat density." It is proved that warm air contains more heat per gram than cold air. When we open the door, and hot air gets mixed up, we increase the heat density in the refrigerator and decrease it in the room.
Radiation is also a small effect at room temperature. It can glow in the infrared. When warm and cold air is side by side or mixed up, they are dipped in each other's radiation.
Warm air has more heat and therefore glows brighter while cold air gains energy and warms up. It loses more power from radiation and cools. So, if we want to keep our fridge cold, we have to avoid letting heat into the refrigerator.