Final answer:
In the heating mode, a heat pump obtains heat from the colder outdoor air, where the refrigerant absorbs thermal energy before being compressed and transferred to the condenser inside to heat the indoor space.
Step-by-step explanation:
In the heating mode of a heat pump, heat transfer takes place at the evaporator where the working fluid, typically a non-CFC refrigerant, absorbs heat from the colder outdoor air (thermal energy). This turns the working fluid into a gas. The correct answer to the question is heat from the colder outdoor air, and not from the indoor air, the ground, or through the refrigerant as such, since it's the medium and not the source of the heat.
After the absorption of heat, the electrically driven compressor then increases the temperature and pressure of this gas, passing it through to the condenser coils inside the heated space where heat transfer occurs: as the temperature of the gas is higher than the indoor air, heat is released to the room when the gas condenses back into a liquid.