Final answer:
Frost formation on an outdoor coil can decrease the efficiency of the system by insulating the coil and hindering heat transfer, necessitating a defrost cycle to restore normal operation.
Step-by-step explanation:
Under certain operating conditions, frost will form on the outdoor coil. This can cause a decrease in efficiency. When the outdoor temperature is low and there is high humidity, the cold surface of the outdoor unit can cause ambient water vapor to freeze, accumulating frost on the coil. This layer of frost acts as an insulator, reducing the coil's ability to transfer heat.
As a consequence, the heat pump or air conditioning system has to work harder, which can significantly reduce efficiency and increase energy usage. To restore efficiency, the system must enter a defrost cycle, momentarily reversing the flow of refrigerant to melt the frost on the coil. Therefore, the correct answer is b. Frost - decrease.