Question

which is a source of thermal energy for a heat pump?
air
fire
sunlight
coal

Explanation:

A heat pump is a device that transfers heat energy from a source to a destination. Air is a common source of thermal energy for heat pumps. Fire, sunlight, and coal are not typical sources for heat pumps in the context of their general operation. Fire is a combustion - based heat source which is not how heat pumps usually extract heat (heat pumps use a thermodynamic cycle to move heat). Sunlight is harnessed by solar panels (photovoltaic or solar thermal collectors in a different way than a heat pump's operation). Coal is a fuel for combustion to produce heat, not a source for a heat pump's heat transfer mechanism.

Answer:

air