Final answer:
Frost accumulation on the outdoor coil of a heat pump in heating mode can occur when the ambient temperature is 45°F due to the principle of heat transfer.
Step-by-step explanation:
When a heat pump is in the heating mode, frost accumulation on the outdoor coil can occur when the ambient temperature is 45°F due to the principle of heat transfer. In this mode, heat transfer Qc occurs from the cold outdoor air to the working fluid in the outdoor coils, turning it into a gas. As the outdoor air temperature drops, the evaporator coils become colder, causing moisture in the air to freeze on the coils and form frost.