Question

which uses combustion by moving pistons, which respond to a change in pressure?
heat engine
furnace
refrigeration unit
heat pump

Explanation:

A heat engine uses combustion. In a heat engine (like an internal - combustion engine), fuel is burned (combustion). The gases produced by combustion expand, creating a change in pressure. This pressure change causes the pistons to move. A furnace is mainly for heating a space by burning fuel but doesn't have moving pistons in the context of using combustion - induced pressure change for mechanical work. A refrigeration unit and a heat pump work on different thermodynamic principles (refrigeration cycle) rather than using combustion - driven piston movement due to pressure change.

Answer:

heat engine