Question

which uses combustion to transfer thermal energy to warm a building?
heat engine
furnace
refrigeration unit
heat pump

Explanation:

A furnace burns fuel (combustion) to produce heat which is then used to warm a building. A heat engine is mainly for converting heat into mechanical work. A refrigeration unit is for cooling. A heat pump moves heat from one place to another (can be used for heating but not via combustion).

Answer:

furnace